Last week, Engadget confirmed that the T-Mobile model of the iPhone 5 -- which will be available unlocked from Apple -- carries the same A1428 model number as the current AT&T / GSM domestic model.
There is a critical tweak, however, for the new carrier: the iPhone 5 now will support the 1700 MHz HSPA+/AWS uplink band, which will enable higher performance on T-Mobile's UMTS Band IV network. Older GSM iPhone 5 units cannot get this fix via software; it's a chip change.
While this means that iPhone 5 and 4S handsets unlocked and moved from AT&T to T-Mobile can never get the highest performance on the UMTS bands, there's hope; the iPhone 5 LTE support for AWS will work just fine on T-Mobile. Now there's just the little question of building out that LTE network (see below).
In contrast to the other three US cellphone carriers, T-Mobile has moved to overhaul the most-despised feature of a phone plan: the contract. New subscribers have the option of buying the iPhone outright (starting at US$579 for a 16 GB model) or a $99 / $199 / $299 immediate charge, just like the other carriers, followed by a payment plan of $20 a month for 24 months.
At the end of that payback window, however, that $20 comes right off the top of your bill -- the phone is yours, so you don't have to keep paying for it. (Try asking for that deal with Verizon.) Of course, if you do decide to leave T-Mobile with the iPhone before the two-year window is up, you still owe the remaining balance of the purchase cost -- but you don't pay a separate early termination fee (ETF).
Regarding 4G and LTE, T-Mobile's US network uses UMTS / HSPA+ "3.5 G" to achieve download speeds ranging up towards the 10 Mbit/sec threshold -- real-world performance may vary. That doesn't really compete with the observed or best-possible LTE performance on Verizon, AT&T or Sprint (yes, there are spots in NYC where you can get 40+ Mbit/sec download speeds).
If you're in the mood for crazy-fast LTE wireless on T-Mobile, you have to wait. T-Mobile's LTE buildout is covering only seven cities so far, with more coming this summer and fall. Interestingly, the specs page for the iPhone 5 on T-Mobile's site only mentions GSM and UMTS (in the I and IV bands), with no discussion of LTE. That isn't a reflection of the actual hardware capabilities, but more likely a marketing consideration -- poor form to push LTE capability as a feature only available to a few customers.
If you're considering a T-Mobile phone, check your coverage before you buy and let us know about your shopping experience! Theoretically, you can buy a T-Mobile iPhone 5 and use it with LTE on almost any carrier in the world, which should make globetrotters very happy.
Updated to clarify that the phone purchase plans do carry an initial charge.