Just a few days ago, rumors started circulating the web claiming that the term "iPhone" was to be dropped from T-Mobile's marketing ploy, and this photo of a T-Mobile flyer confirms them. Did Apple ask T-Mobile to alter the campaign's nature, or was it the carriers? We don't really know.
However, T-Mobile would still let any owner of an unlocked AT&T smartphone to join its network. In fact, anyone who joins the party might potentially save up to $50 by switching to a T-Mobile plan. As far as iPhone 5 owners are concerned, they are more than welcome to switch to T-Mobile, as Nano-SIM cards are in stock at the carrier's outlets.