The A1428 edition of the iPhone 5 that Apple will start hawking on April 12th (in lockstep with T-Mobile's launch date) will arrive unlocked out of the box. By default, Apple will sell these at full MSRP in a "SIM-in unlocked fashion," as confirmed to us by an Apple representative here at T-Mobile's event in New York City. Of course, those wishing to pay T-Mob's advertised $99.99 up front price will also be able to do so right at an Apple store, but eager jetsetters that are simply looking for a frictionless way to purchase an unlocked iPhone 5 that works on both AT&T and T-Mobile's LTE bands have but a few weeks to wait.
Apple is said to be moving to the new model for AT&T sales as well, so presumably after April 12th an iPhone purchased from AT&T will support AWS. Units purchased from T-Mobile have been verified to work on AT&T's LTE network; however, you will need to have the device unlocked before you can switch over.
Sales at the Apple Store are scheduled to begin at 12:01 AM on April 12th.
*According to Wikipedia, networks that use AWS in the United States include: T-Mobile, Big River Broadband, Cincinnati Bell, Mosaic Telecom, and Cricket Wireless. Networks that use AWS in Canada include: Wind Mobile, Mobilicity, Videotron Mobile, and SaskTel. Support for any other network than T-Mobile has not yet been officially confirmed.