Rumor has it that T-Mobile will start selling the iPhone later this year. "Talks are apparently in an "advanced stage," and the source puts the odds at about 80 percent that it's going to happen".
Seems like there would be some issues though:
With T-Mobile there is one big problem, and that's their 3G frequencies. They operate on a non-standard 1700/2100 MHz band, whereas AT&T uses the standard 1900MHz for 3G with 850MHz in certain markets. That means the 3GS is not going to work on T-Mobile.
The iPhone 4 works on 2100MHz, but it is unknown if 1700MHz support was added as well as no testing has been done. This would be the biggest hurdle that would need to be crossed in order to make this even a remote possibility.
If you're with T-Mobile would you want to get an iPhone? Either way it should be interesting to see how this develops.