You can't BUY an AT&T compatible GSM iPhone from any other carrier. This is the only other Apple Recognized GSM carrier in the US. T-mobile shares GSM, but they don't have PERMISSION from Apple to sell or support iPhones.
So while you CAN move an out of contract iPhone to Tmobile, lots of features don't work.
I don't think that was my point. My point was you can get an iPhone on Verizon, Sprint, etc. So the iPhone itself is not an ATT exclusive - and hasn't been for awhile now.