Cult of Mac seems to be very confident that T-Mobile will be getting the iPhone in the third quarter of 2010. Since AT&T and T-Mobile are both GSM networks, it wouldn’t be too much trouble for Apple to make it work, so it is certainly plausible. (And they wouldn’t be the first either.)
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.
AT&T has been the exclusive carrier of the iPhone for far too long, and it will be a relief once another carrier gets ahold of it. Verizon and Sprint have been backing Android pretty heavily, so if Apple’s contract with AT&T ends, T-Mobile may be the only realistic choice. We’ll see in the coming months.