Pristine Angel And Equipment

[Zhalfirin]

So let's say I attack with my Pristine Angel.  During my post-combat main phase, I then equip my Angel with Skullclamp.  I then cast some other spell, choosing to untap my Angel.  Does the Skullclamp fall off?

My gut tells me yes.  Moreover:

420.5k - An Equipment that equips an illegal or nonexistent permanent stops equipping that permanent but remains in play. [CompRules 2003/10/01]

I assume the Angel becomes an illegal permanent when it gains protection from artifacts?

[Jebus]

It falls off.

A creature with protection from Artifacts cannot be equipped.  The Equipment is equipping an illegal permanent, so it falls off.