Cult of Mac is claiming an exclusive, highly placed source has told them iPhone 4 is coming to T-Mobile US as soon as Q3:
Talks between Apple and T-Mobile are at an advanced stage, our source says, and it’s 80 percent likely that the iPhone will be coming to T-Mobile in Q3.
The source works at T-Mobile but asked not to be quoted directly and to remain anonymous because they aren’t authorized to talk to the press.
T-Mobile is a GSM carrier, like AT&T, but they use a 3G band probably not even supported by iPhone 4's 5-band radio (though rumors suggest Apple might simply have chosen not to list it). If it's not supported, Apple will either have to add that band to future units, or produce a new model just for T-Mo.