So for those of us keeping count...

Leyline is almost a 100% go main deck of manaless Ichorid.  Ichorid has the ability to run four of the card, filter if it were to ever actually be drawn later, and to utilize the card in other ways later in the game (i.e. paying the alternative casting cost to Unmask). 

5 color Uba stax, Uba Stax, and5 color stax could incorporate the card into their main decks, as it is a 40% chance to knock out an opponent's yard turn 0 for 0 mana, and also because they have an effective filter mechanism. 

To continue the discussion...

It may just be me, but the sense of the entire format seems to be changing.  For the first time in a long while we see a deck that is totally void of power, blue, and artifacts running rampantly unchecked in our format (Ichorid).  The deck plays spells because it can, not out of necessity.  It could theoretically never play a spell, and consistently win by turn 3.  Now obviously this is a single case and not every deck in the Meta game fits this mold, but I believe that we could be on the verge of a fundamental shift in the way vintage magic is perceived.  At the forefront of this change have been decks that utilize all resources effectively.  For the last 2ish years stax was the Meta game that had to be beat.  Why?  Because of its amazing ability to generate fat mana, and weld fatter artifacts into play for free (not to mention never miss a land drop thanks to Crucible).  It soon became apparent that Bazaar of Baghdad is an amazing filter, giving both new cards in hand, and filling up the graveyard with things that could be "gimped" into play. 

Now correct me if I'm wrong, but in current vintage magic don't all decks, in some fundamental way, utilize their graveyard in a similar way? 

Control Slaver:  Big on welders, has Thirst for Knowledge as a filter, loves that Yawgmoth's Will, all about setting up good board position.

Gifts:  What's recoup, Yawgmoth's will, and every other card your opponent sends to you graveyard going to do RFG'd?

All stax variants:  As discussed above, Crucible/Welder is almost what makes these decks in current builds where lock components and fat drops are at an all time low; thanks to our friend Bob the confidant.

Tendrils combo:  This one is arguable... I don't suppose that graveyard manipulation technically effects their ability to play nine spells and have 2BB mana remaining to tendrils for the win, but how less broken is Will in this case... and with a newer trend of playing Bazaar in these decks in order to optimize Will, the graveyard is becoming all the more important.

Bomberman, Oath, Dragon (yeah I know, who plays dragon anymore, besides me  lol...):   All have fundamental ties to their graveyard.

Except for U/W fish and EBA these have racked up huge numbers of top 8 finishes at some of the most prestigious vintage events in the world. 

What really determines what justifies a main deck slot?  I suppose in theory it has to fall into one of 6 categories...

First, every deck needs some way to win...  I've heard it said that really no deck, aside from aggro (who really wants to pay mana for a creature that does little more than attack, when they can much easier play an Oath of Druids and get a 6/x Swiss army knife or a Tinker out an 11/11 whoopin' stick and win) can afford to have more than 4-5 slots in a 60 card deck dedicated to winning.  This would include your Tinker/DSC, Burning Wish/Tendrils, Dragons/Animate effects, Trikes, and heaven help us for even recalling the days of Decree of Justice and Morphling. 

So that leaves us with 55-56 slots. 

Most vintage decks run at least 5x Moxen, 1x Lotus, 1x Sol Ring... while most (stax, gifts, cs, and Combo) run a full assortment of Solomoxen, crypt, vault, petal (in some builds) along with 4-6 fetch lands, 3-4 basic lands, 5-6 duel lands, and 2 or 3 utility lands (Factories, strip effects, ports, etc...).   So that's any where from 7-10 artifact mana and 14-19 lands taking up 21-29 slots for mana generation.

So that leaves us with 27-34 slots.

Then we have the arm long list of restricted cards that almost every deck utilizes... Your Demonic Tutor, Vampiric Tutor, Mystical Tutor (excluding Stax), Imperial Seal, Ancestral Recall, Time Walk (excluding stax), Fact or Fiction (just as an example, Gifts is probably run more now days even in CS, but then again Gifts isn't restricted so nah =p), Tinker, Yawgmoth's Will, then especially in combo Necropotence, Yawgmoth's bargain, Regrowth, Memory Jar, Wheel of Fortune, etc...

This leaves us...well I don't know, I kind of lost count...but leaves us with each deck's particular staples, somewhere between 21 and 28 slots for our Brainstorms in anything with Islands Dark Rituals in combo, our Mana Drains in control, our lock components in stacks, and our Merchant Scroll type stuff in Gifts...

The 4-8 slots for standard proactive disruption: Duress, Force of Will, and Xantid Swarm.

And finally we have the 5 or 6 cards that are totally up to your personal playing style...about the only 5 or 6 cards that you see varied from the standard archetype at major vintage events.  You know the ones like Rack and Ruin, Darkblast, Fire//Ice, God's Eye (inside joke), the cards that you sit around and bust the last brain cell you had left after 8 rounds of Swiss debating one over the other before the next tournament.

Now, which slot would Leyline fall into?  Obviously it isn't going to swing for the win, it doesn't tap for mana (although in mono black and Ichorid, where we've already discussed it's viability, with unmask it does pitch awfully well), it doesn't find us our win or the card we need to get out of our current situation, it doesn't tap/sac to give us 7 new cards like Jar, it doesn't help our storm combo or break or effectively improve our hand, game, or board position in anyway whatsoever. 
Perhaps these facts are what, in higher schools of magic theory, permanently brand the mark of side board only on to the card.  But we do still have the final two categories, and it's here, in these last 2 slots, the 9-14 cards, dedicated to proactive disruption and personal playing style, where I feel this card earns its stripes. 

I don't think there are many people who would argue against Duress being a firmly established icon in the world of disruption.  Turn one this card can lay to waste the most elaborate of dreams of glory in a magic player's mind.  What does this card do?  It removes one non-creature, non-land card from an opponent's hand and puts it in their graveyard (thus taking it away from their arsenal), and it has the added effect of giving you the knowledge of what exactly is in their hand.  Very solid.  But this card, aside from the times that it is Forced, through the alternative casting cost, really doesn't provide any sort of card advantage, usually isn't that useful when cast after turn 3 or 4 (except to protect your combo when going off), and after turn 5ish is, most times, a dead draw.  These points are exactly the arguments against Leyline that I've seen brought up, but no one really questions Duress's place of power in the main deck (and most of our most established archetypes today have experimented/had favorable results with the card in the main).  In opposition to Duress's not creating card advantage, Leyline does exactly that.  Every deck that has dreams of sending cards to the graveyard to Will, weld, play through Crucible, that they've discarded to Thirst, discarded to Tog, dredged, or discarded to Bazaar is gone, it makes cards without buyback (who plays buyback anyway) one time use only.  I believe this creates not only actual card advantage (removing at least x cards from the game through the use of 1 card), but also virtual card advantage (in the same way that Chalice of the Void set at a number where your opponent will feel the brunt of the effect).  In every other aspect it functions in a fundamentally similar way as Duress, it makes spells that an opponent had access to inaccessible. 

In response to sys41o:  It really is a Meta call.  It wouldn't even be conceivable to play this deck in an aggro heavy environment, but I really wanted to look at this subject from the point of view of large tournaments where the top 8 is almost always entirely comprised of Tiered decks.  Also, as mentioned above, it is entirely better if this is dropped turn 0, just like most other forms of proactive disruption (Chalice for 0, Unmask, Duress, etc...), but even later on when cast it hinders in a similar way.  Yes it only slows your opponent down, but I think slowing your opponent down while you set up better board position is the very definition of tempo and board control.  Every other form of proactive disruption in the format, in and of itself, isn't enough to win the game, but what it does do is tilt the scales in your direction.  I don't think any stacks player plays turn one 3-sphere and expects that one form of disruption to carry him/her to victory, but it gives you an advantage, that when played upon leads to victory.  And while I agree that decks seldom win through graveyard manipulation alone, you would have to concede to the points listed above concerning the current tiered decks in the format.  Each and every one of them utilizes their graveyards in ways fundamental to the deck's success.

Another thing to keep in mind is that all of the decks we're talking about (the ones I perceive as "tiered" decks and thus the litmus test by which other decks must be measured) all have ways to filter through late game draws...bazaar, Thirst for Knowledge, Brainstorm, etc. 

To meadbret:  I think this card is more castable than we give it credit for.  I don't think there are many of us who will necessarily mulligan until this card is in our opening hand (unless its past game 1 and we know our opponent is playing Ichorid), but 2BB isn't awful (okay it is awful in general) to come up with.  Several games I've seen it hard cast turn 2 by B/R Stax and 5 Color Stax.  And I really don't think there are that many matchups where it is truly dead.  In a tiered environment what matchups is it dead against?

To Harlequin:  I agree 100% with the first half of your post.  I think that your analysis is right on.  As far as your views on Planar Void, I’m afraid I don't share your views.  Now of course in a non-powered (excluding manaless Ichorid), non-tiered environment this whole discussion is totally pointless.  In other decks besides stax variants, CS, Gifts, and combo variants the graveyard is a second class citizen hardly worthy of paying any heed to, but take it from one who thought along your lines that Planar Void could  be a substitute for Leyline...it doesn't work =(.  I had to discover this tasty bit of intelligence from a beating from Robert Vroman.  I was playing CS at the time and he was playing his trademark Uba Stax.  Game 2, with the intentions of nullifying his welders, his draw engine, and his Crucible I sided in 3 Planar Voids.  What I hadn't come to realize is that in the process of doing this I had inadvertently cut off my welders, my thirsts, and my Will.  Also, as you mentioned it is a triggered effect that can be played around by the common cards included in most of the decks it is designed to hinder.  As far as you other alternatives, I agree that they are comparable in effect, but they're hampered by their magnitude.  They are far less effective.

I guess all of this discussion really leads us back to the original question, is the card good enough for a main deck slot?

I still believe it is awesome as a proactive disruptor.  Having a 40% chance of having it in play turn 0 is admirably strong.  Most tiered decks are sufficiently suppressed by it for serve its purpose of establishing good board position and staying alive long enough to win yourself.  And, as I believe the entire format is truly waning away from several of the time honored aspects of the format (moxen, the incredible skill level it takes to compete at top levels, and as was evident by the rise of stax the shift from conventual magic play).

I would like to hear more of your input on this subject, especially in relation to this card in stax variants, CS, and Fish. 
I don't think that Gifts would have room for it, nor does it really care about the kind of disruption this card creates on its opponent.

Theoretically speaking, what cards could these decks part with to include Leyline?

In a tier 1 setting, is Leyline of the Void good enough for a main deck slot in many of today's tier 1 decks?

As far as match ups go the only real bad match up with this card main deck is U/W fish, but even against U/W fish it can be effective as a way to starve Grunts. 

My friend is a fervent player of Scuba Stax, and the other day while we were playing I tossed out the idea of main decking Leyline.  We both initially chuckled at the idea, but upon consideration engaged in deep discussion of the idea.  At the time I was playing manaless Ichorid, and had 2-0'd him largely due to a pair of Leyline filled opening hands (plus the fact of us not side boarding between the two games). 

Leyline is obviously good against manaless Ichorid, and is even good against colored Ichorid.  Also, as I discovered in our games, against stax is definitely solid.  Control Slaver and Gifts generally have bounce, but it is a good tempo boost, while they find a way to bounce it.  Dragon and all builds of Oath probably fear this card as well.  Also, it is fairly suppressive against many storm based decks. 

With as many good match ups as this card has, it seems as a possible main deck slot in any deck that can support it (definitely any deck that wouldn't be opposed to mulling into it, although the odds of that are slightly unfavorable). 

Anyway...for argument sake, what decks could consider main decking it, what cards would they cut for it, and in the end is it a viable option?

Thank you guys for your help.  I think we can safely say that Cranial Extraction is either unnecessary, not cost efficient, or an inferior version of cards currently available in the format. 

I did see one card from the new set.  A split red/red card.  the first half is a shock (even at instant speed, aren't we special) the seond half is 2R return target creature an opponent controls to owner's hand, also at instant speed.  They're just full of surprises aren't they.

Any card that has the potential to rob an opponent of every copy of a given card in his/her deck peaks my interest.  The question that must be asked though, is Cranial Extraction playable, is the one that I now pose?  Several cards have amazing effects, but just fall into the realm of being completely unusable in the format for one of many reasons:  casting cost, speed, or possibly the existence of a better version of the card.

First of all let us look at the casting cost of this spell, 3B.  It goes against one of the primary casting cost rules of Magic, it costs more than three and is not blue or an artifact.  By this criteria this card is unplayable, but there are other cards that fall into this category that still see play, such as Yawgmoth's Bargain, and Nether Void (I guess we have to be very loose in our definition of the term "sees play"), .  Now by no stretch of the imagination could the power of this spell be in anyway compared to that of Bargain, but it may have an effect that could be in the same ball park as Nether Void.  Although it breaks this long tried and tested fundamental rule of vintage magic, I still believe that an argument exists for it.

As for the speed of the spell, sorcery, we could ask for better, I don't think we'd ever get it, but one cardboard nerd's dreams aside, decent for the established guidelines of the game.

Finally does a better version of the card exist?  I would like to say arguably no, but at the same time yes.  Extract:  U sorcery search target player’s library for a card and remove that card from the game. Then that player shuffles his or her library. In a world of deck lists that go on and on for 15 pages because of the gaggle of one ofs in the vintage Meta game some would say that extract is enough, but do these staples of which we speak truly define any deck?  If we were to cut one of them from a deck, would it really make that much of a difference?  I cannot come to a definitive answer, but what about the cards upon which the decks are built, the mana drains, the smokestacks, the goblin welders, the dark rituals, the worldgorger dragons, etc....  In one fell swoop it is possible to completely eliminate everyone from an opposing deck. 

It is that possibility that makes me very interested in seeing if there is any potential for this card in vintage.

For the sake of argument I threw together a deck for discussion.

Sorceries   

4 Cranial Extraction
1 Demonic Tutor
4 Duress
4 Night’s Whisper
2 Tendril’s of Agony
1 Yawgmoth’s Will

Instants   

1 Vampiric Tutor
3 Cabal Ritual
4 Dark Ritual
3 Diabolic Edict

Enchantments   

4 Phyrexian Arena

Creatures   

4 Withered Wretch

Non-Basic Lands   

3 Polluted Delta

Basic Lands   

7 Swamp

Artifacts   

1 Black Lotus
1 Mox Pearl
1 Mox Sapphire
1 Mox Jet
1 Mox Ruby
1 Mox Emerald
1 Sol Ring
1 Mana Crypt
1 Mana Vault
1 Lotus Petal
4 Pithing Needle

The deck is mono black; though blue could be splashed in for total uberness, and it would in theory seek to disrupt the first couple of turns, while developing a strong hand/graveyard, then tendrils for the win. 

The bad news is that in theory it would be slower than your traditional grim long, IT, TPS, or what have you, but you could trade some of that speed, for a little more stability and a lot more disruption along the way.  Then again I'm not trying to break the format with a new deck, just posing it for the consideration of Cranial Extraction.  Anyway, let me know what you think, the good, the bad, the ugly...

Thanks