AT&T has stated if you bought your iPhone after May 27th, you can swap out your iPhone for a free 3G iPhone upgrade - no cost, no contract extension.
Of course, if you bought it before that, you get to sign a whole new 2 year contract, extended from where yours ends now to grab a 3G iPhone. Unless you want to pay full price, of course, like us T-Mobile folks are going to have to do.
Gizmodo