This is evidenced by a new TV commercial that makes fun of Applu that it still works with AT&T's "outdated 3g network" and not the fast 4g provided by t-mobile. However, the fact that the only iPhone 4 provider will be exclusively AT&T in the near future does not change that. The whole advertisement is carried in the spirit apple Mac vs Pc ads. You can see the ad here:
https://youtube.com/watch?v=3KmfXupi9cg
here: where? :-) you see tmobile has it here and for a change it is just building an outdated 3g