Last Updated: Sept. 20, 2012
While a T-Mobile iPhone is certainly possible, one thing is keeping the iPhone from coming to T-Mobile: Technology.
After Verizon first got the iPhone, it was widely assumed that it had negotiated some window of time during which another iPhone carrier wouldn't be added. That seemed to be confirmed because Sprint didn't begin to offer the iPhone until roughly 6 months after Verizon. Now that there are three major carriers in the U.S., and many smaller ones, it seems likely that there are no business reasons preventing T-Mobile from offering the iPhone.
The bigger reason is that the technology in the current iPhones won't work with T-Mobile's network. To bring the iPhone to Verizon, the challenge was great: AT&T and Verizon don't use the same kind of network, so Apple had to develop an iPhone with a different kind of wireless chip.
AT&T and T-Mobile's networks are similar--both use the same basic technology, GSM. However, AT&T's phones operate in the 850Mhz and 1900Mhz parts of the 3G spectrum, whereas T-Mobile's work in the 1700Mhz and 2100Mhz bands. So, to make an iPhone for T-Mobile, Apple would have to add support for those bands to its iPhone chips. To date, it hasn't done that.
iPhone for T-Mobile Release Date
For a time, AT&T was expecting to buy T-Mobile, which would likely have included a merging of their networks and support for the iPhone on T-Mobile. However, that deal fell through and T-Mobile remained an independent company.
At the iPhone 5 announcement, T-Mobile was nowhere to be seen. However, the company does offer an option to owners of unlocked GSM iPhones that will allow them to be used on T-Mobile's network. If you want an official iPhone for T-Mobile, though, it appears you'll be waiting until at least 2013.