Shapeshifter + Dreadnaught

[Trix]

If Player A has a Phyrexian Dreadnaught as the top card in his graveyard, and he plays a Vorath's Shapeshifter.

Question: Does he need to sacrifice creatures due to Phyrexian Dreadnaught's ability?

[Jebus]

No, you will not.

The Shapeshifter does not gain the text of the dreadnought until after it has entered play.

The replacement effect of the Dreadnought is applied before it ever enters play.  The Shapeshifter won't have the replacement effect text at the time the effect woudl be applied.

[Lockdown]

I'll admit this is a complicated question that I don't have an answer to.  Using the links in my Rules FAQ above, I discovered the following text, but it is confusing:

//Text on Shapeshifter
-If the top card in your graveyard is a creature at the time this card comes into play, it comes into play with the appropriate characteristics of that card and will trigger any of its own (or others) comes into play abilities as appropriate. [D'Angelo 1999/06/01] It will not apply any "as comes into play" text on the creature card, however. [D'Angelo 2001/08/31]

//Text on Dreadnaught
-Text (MI+errata): 12/12, Trample. ; If ~this~ would come into play, sacrifice any number of creatures with total power 12 or greater instead. If you do, put ~this~ into play. If you don't, put it into its owner's graveyard. [Oracle 2000/02/01]

//Rule 503
-503.5 - An object that comes into play "as a copy" of another permanent becomes a copy as it comes into play. It doesn't come into play, and then become a copy of that permanent. If the text that's being copied includes any abilities that replace the comes-into-play event (such as "comes into play with" or "as [this] comes into play" abilities), those abilities will take effect. Also, any comes-into-play triggered abilities of the copy will have a chance to trigger.

The last bit of text is what makes me inclined to agree with Jebus, as it says it "becomes a copy as it comes into play", which seems to mean that it is already on its way into play, and the replacement effect does not trigger.

Edit:
Ah, my mistake Jebus.  I missed the text on the shifter:
This card is no longer a copy card (see Rule 503). It now simply sets its own characteristics based on a characteristic setting ability (which is a special kind of continuous effect). [D'Angelo 2003/09/08]

However, this still confuses me, what is the exact wording that makes you think that shifter doesn't trigger?

[Jebus]

Actually, Rule 503 does not apply here at all as the Shapeshifter does not have a copy effect.

If it did have a copy effect, like Clone, then you would have to sacrifice.

since it doesn't, you do not sacrifice.

[DEA]

trix, are you charles?

[Jebus]

However, this still confuses me, what is the exact wording that makes you think that shifter doesn't trigger?

Well, there is nothing to trigger at all, as there are no triggered abilities.

It does have a replacement effect that occurs as part of the event that puts it into play.

The Shapeshifter's ability does not get any text from the Dreadnought until after the Shapeshifter actually comes into play, which is long after the time to apply the Dreadnought's repalcement effect.

[Eastman]

There is no question about this. You do NOT need to sack anything, barring additional effects.