I’m not sure the size of the SIM has anything to do with carriers. They presumably just want to safe some space in the new iPhone. (And if you introduce the Micro SIMs in the new iPhone you at least want to be consistent and introduce it everywhere.) Such transitions are not uncommon – SIM cutting was already a sport a few years ago. (You had to cut down your SIM card from credit card size to the current size.)

That said, aren’t you Americans always gonna be out of luck? I thought the iPad’s 3G only works with AT&T, the best you can do if you don’t want AT&T is get Edge and T-Mobile. That must suck.