You could always use your phone on T-Mobile aslong as it was unlocked. You just didn't get 3G service.
I've heard people saying you could now choose T-Mobile as your iPhone 5 carrier. Is this true?
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.