Deutsche Telekom CEO Rene Obermann just stated in an investors' day meeting that T-Mobile USA will be launching Apple products in 2013, and speculation is heavy that that will include the iPhone.
No details are available yet, but in order to get the iPhone successfully up and running on T-Mobile USA, Apple would have to add support for HSPA on the AWS spectrum, or T-Mobile will need to greatly increase their 3G support on the non-AWS spectrum, or both. T-Mobile has been deploying iPhone-friendly 3G in several markets this year, so it could well be the latter.
And that's just 3G. They still haven't even left the gate when it comes to LTE. T-Mo's merger with MetroPCS could give them the resources they need to begin building out LTE, but Verizon, AT&T, and even Sprint certainly aren't going to make catching up easy.
So, there's a lot of work ahead. But hey, after 5 years and pretty much every other carrier, it's a start, right?