Not gonna happen. T-Mobile's 3G will be on 1700/2100MHz, while AT&T's is on 850/1900MHz. And I don't think Apple will add extra bands for a carrier that it doesn't support.
Apple will have to add bands anyways EU uses 1900/2100 for 3G while we use 850/1900 so there is a good chance the 3G portion would work on T-Mobile. Otherwise they would have to make 2 (or more) different phones, 1 for the US and 1 for EU and anyother countries that use different bands.
EDIT: hmm I can't read, lol yeah t-mobile 3G wouldn't work, but HSUPA technically could work as would LTE.
That is why 4G is going to be so great. Everything becomes IP based, so it doesn't matter how you get on the network all communication will be done via IP. The underlying connection becomes less important.