Starting April 12th, Apple retail shops as well as Apple's online store will begin selling the new and gently tweaked A1428 model of the iPhone 5. For those paying attention, that's the same model number as AT&T sells today, but Apple's enabling support of the AWS bands from the factory on the new guy -- and sadly, Apple affirmed to us that it's not something that can be enabled via a simple software update for A1428 iPhone units already in circulation. To say that another way, existing iPhone 5 owners on AT&T cannot simply apply a software patch to have AWS HSPA+ 42 support added. You'll need to buy a new phone next month. (Note: AWS LTE does and will indeed work, whether you have the current, AT&T iPhone 5 or this new, updated model. It's only HSPA+ 42 that is lacking on the current model.)
That said, 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.
Oh, and yes, we confirmed with Apple that the new A1428 will indeed support AT&T's LTE network. In other words, your T-Mobile iPhone 5 will run uninhibited on AT&T's LTE network if it's unlocked. To boot, Apple is quickly phasing out the existing A1428 hardware, and will soon replace all of them with the new, AWS-enabled model. In theory, that would mean that iPhones purchased through AT&T (after April 12th) would also ship with the appropriate firmware to let AWS support run free, but of course, then you're up against AT&T's far less friendly unlocking policy. For US users anxious to snag a truly unlocked iPhone 5 that'll hum along on pretty much every LTE band in the developed world, Apple informed us that its 24-hour locations will begin selling these promptly at 12:01AM on 4/12. For those who order from Apple's online site, you'll need to phone up T-Mobile to have the unlock applied.
Update: We've updated this post to clarify that the lack of AWS is only for HSPA+ 42. The older model phone phone does not lack AWS on LTE and so will have no problems connecting to LTE.