T-Mobile is the only main US carrier that doesn't carry the iPhone. It's currently unclear as to why Apple hasn't partnered with T-Mobile to sell the iPhone, but the most logical assumption would be that it has something to do with the AT&T/T-Mobile acquisition that is currently being reviewed by the FCC.
The CEO of T-Mobile has gone on the record time and time again to explain that his company really wants the iPhone. And Apple is apparently to blame for all of the unhappy, iPhone-less T-Mobile customers out there...