Firs you'd need to get the phone unlocked so you could use an AT&T SIM card. It will work on GSM/EDGE but it won't work on AT&T 3G. It only has 3G on T-Mobile's 3G bands, not AT&T's.
No GSM phone can work on a CDMA network. That means no T-Mobile or AT&T phone (GSM) will work on Verizon or Sprint (CDMA).
Since the Touch Pro2 will be coming to all 4 major US carriers, I'd suggest you wait until it's available for you carrier.
-------------------- Lisa Gade Editor in Chief, MobileTechReview