A bit optimistic according the T-Mobile folks at their stores. It's more like sometime in the next 6 months. However, this comes from their CEO so must be credible. T-Mobile has to invoke new frequencies to be compatible with the GSM version of the iPhone.
The iPhone 5 (with T-Mo SIM) is already receiving 3G service here in NYC. The LTE bands have yet to roll out, but I suspect that is what Apple is waiting for. As a matter of fact, T-Mo's service here is pretty darned spectacular, and beats out AT&T by a long shot. I've seen unlocked Verizon iPhone 5's with Tmobile SIM's in them getting a ping of 3ms on cellular! Dl speeds are very comparable with AT&T's and that's when in range of an AT&T amplifier.
Selfishly, I hope that the iPhone never goes to T-Mobile. I've been on their network now for the past month and a half, and I've been a lot more than pleasantly surprised by their overall network performance here in NYC. This used to be quite the opposite case, in fact.
I have an HSPA+ equipped phone that consistently gets download speeds between 15-19 Mbps and up of about 2.5 Mbps. And that's just HSPA 21... When they roll out LTE (which my phone is capable of utilizing) that will double.
Plus, I've never experienced better voice quality than now, and I've been with every single major network here. Having iPhone users hog up the bandwidth isn't something I'm looking forward to. Especially since I'm only paying $30 a month for unlimited data! Of course that unlimited service comes with a downside.. 100 minutes of talk time, but I'm able to circumvent that by using Vonage mobile as well as Google Voice.
What ever T-Mo is doing right now, they're doing it right. I can only hope they keep investing more into their infrastructure.
Doug