Ok, I just got done vetting this question with a few IDPA SO's who work Sanctioned matches, spoke to the major aftermarket XD folks, and also passed the equipment check at the 2011 Golden State IDPA Championships this past weekend (4th CDP MM...yeah!)
I run an XD 45 Tac in both ESP and CDP. I have the Canyon Creek EXTENDED mag release. The key to this upgrade is to AVOID the oversized diameter releases, they are NOT IDPA legal. Any of the extended releases available from Springer Precision, Powder River Precision, or Canyon Creek are all legal on XD's in ESP and CDP.
The key is (and this part is eloquently stated in a previous post above) that the mag release cannot extend more that .20 from the "frame" of the pistol. The reality is that the release is recessed on an XD, and most will measure more that .2 from the point where it comes out of the gun. This got confusing for me.
However, because the GRIP on the XD is also considered the frame of the gun (as is the case on all shootable tupperware), you are fine. Just make sure the release is of the same diameter as the stock release, and that it does not extend past .2 from the GRIP of your XD.