They are re-farming their networks so 3G will definitely work with the iPhone 5/T-Mobile (all areas by beginning of 2013). Just can't run 4G though, bummer.
If you buy an unlocked iPhone 5 from Apple, then you can use the device on T-Mobile's network -- they're even willing to get you all set up if you bring your iPhone 4/4S/5 into any of their retail stores.
They are re-farming their networks so 3G will definitely work with the iPhone 5/T-Mobile (all areas by beginning of 2013). Just can't run 4G though, bummer.