Want to be uncritable? Or damn near it? Check out Veriden's post in the Feral forum (http://wow.thedruidsgrove.org/forums/showthread.php?t=12105#post147353) where he goes through some basic math comparing defense vs resilience+agi combination.

The end result? They are near equal. Check out the post for some example on how to achieve the said "crit cap."