didn't at&t go back and forth on that with the iphone? If you have an iphone on the AT&T network, they want you to have one of their iphone data plans.

To answer one of OPs questions, if it's unlocked, it will work on AT&Ts network (with an AT&T sim) or on T-Mobiles network in the USA (with a t-mo USA sim). However, if you use it on t-mobile in the USA, you should turn off 3g, since the iphone doesn't use t-mobile's USA 3G frequencies. It will work using GSM/Edge/etc just fine in that case. (That's what I do, but I don't have a data plan. I only use wifi for data.)

-David