T-Mobile did not forget about their customers already using unlocked iPhones on their network. The carrier plans to send out a minor iOS software update that will enable new features like LTE, Visual Voicemail and other "Network/Device optimizations" that are not available at this time. Anyone using an iPhone on the T-Mobile network with iOS 6.1.x or higher will receive the iOS update on April 5.
Previously T-Mobile customers could only use unlocked iPhones on the T-Mobile network, but that has all changed since T-Mobile has now become an official Apple carrier. T-Mobile customers will be able to buy the iPhone 5 (and iPhone 4 and 4S in select markets) starting on April 12. T-Mobile has also stopped subsidizing phones, meaning customers will still have to pay full price for any devices bought directly from the carrier.