I've been a happy T-Mobile customer for many years and always said I would buy an iPhone as soon as it came to T-Mobile. According to some photos and information on Boy Genius Report Apple is testing a white iPhone with T-Mobile 3G bands. However, with the AT&T and T-Mobile deal being worked out, this may simply be a test platform since it doesn't seem to make much sense that Apple releases a T-Mobile USA only 3G version of the iPhone.
If the AT&T/T-Mobile deal goes through, then T-Mobile customers will eventually get the iPhone since they will then be AT&T customers. It would be awesome to see Apple release an iPhone 4S or iPhone 5 that is penta-band 3G, like what Nokia has done with its latest Symbian smartphones. No one else has yet released a penta-band 3G device that lets consumers use their GSM device with 3G data around the world without worrying about specific carrier frequencies. Like other things Apple has done (video calling), they could release a penta-band iPhone and say they were the first to do it too :)