T-Mobile already "announced" that they would begin receiving Nano SIMs in preparation for customers wishing to use an iPhone 5. Well, we've got a bit of double confirmation for you thanks to this this internal screenshot which highlights the different between the three types of SIM cards now in use or set to be used by T-Mobile customers. As you can see, the Nano SIM is exclusive to the iPhone 5 as no other T-Mobile phone uses it. For that matter, no other phone in the US uses a Nano SIM.
Apple is not yet selling unlocked iPhones in the United States but they will soon be available starting at $649.