With T-Mobile, just change phone, as long as it works for you, it works for T-Mobile.
Not trying to make an argument out of it or anything :) but I figured since you're a T-Mobile customer, you're already in for a rough ride :( Thought I'd try to offer some consolation as we welcome you to the support group... I mean, network. Yeah, network.
What type of restriction did you encounter with the iPhone. Let's say I buy an unlocked iPhone (from France for example), and then, add the AT&T simcard, would it work?