Right now, there are small pockets of t-mobile coverage where iPhone users can get 3G. Very small pockets, but hopefully they will be expanding pockets soon.
But if you're a T-Mobile user that wants to go iphone, you either need to get really good at holding your breath and crossing every appendage you have or just go ahead and switch somewhere else.
Honestly, if the iPhone this year doesn't get tmo bands, I'd give up any hope at all.