As a T-Mobile user in a contract, this kinda hurts. With T-Mobile refarming to 1900MhZ to offer 3G compatibility to iPhone and over a million unlocked iPhone users on there network, I'm very surprised at the lack of an official T-Mobile iPhone.
My guess is you won't see a iPhone on T-Mobile until they are finished rolling out the refarm or if the next iPhone supports 3G over AWS.