updated 06:30 pm EDT, Tue March 26, 2013
Requires different radio, so previous units can't update to it
[Update: AWS iPhone 5 has cleared FCC approval] In light of the rollout of Apple's iPhone on T-Mobile, Apple has announced that it will begin selling a very-slightly modified version of the iPhone 5 starting April 12. The "new" version, which will carry the same model number, is different only in that it adds capability for T-Mobile's LTE network frequencies, known as AWS (Advanced Wireless Services). No other new features have been added, but because the AWS addition requires a hardware change in the cellular radio, current iPhone 5 units cannot be upgraded through software to AWS compatibility.
The tweaked A1428 model will become the default GSM version of the iPhone, sold through Apple's online store, AT&T and T-Mobile. The unit will also now ship factory unlocked, meaning a tweaked iPhone 5 bought at full MSRP from Apple would also work on either T-Mobile or AT&T's network. The practical upshot of this is that current iPhone 5 owners who want to use T-Mobile's version of "4G" LTE will need obtain the new model, as the existing iPhones can't be upgraded through firmware or an iOS patch.
T-Mobile does have a "trade in" program, but specific details on "upgrading" existing iPhone 5 units to the AWS-compatible model haven't yet been made available. It should be noted that buying an iPhone from T-Mobile at the $100 upfront price from either the carrier or Apple will result in a carrier lock on the iPhone until the cost of the unit is fully paid.
T-Mobile's current plans call for a $100 fee plus a $20 per month payment on the iPhone 5 separate of the chosen talk/text/data plan options. However, users can pay off the cost of the iPhone at any time they wish, and T-Mobile will remove the carrier lock immediately.
According to a report from Engadget, Apple's 24-hour stores will being selling the AWS-friendly iPhone 5 promptly at midnight on April 12. The company says it work to clear all existing stock of iPhone 5 units in advance of the tweaked version's debut. The updated iPhone 5 should now be compatible with almost every LTE band frequency around the globe, and even the current model will work on nearly all GSM 3G networks and most LTE bands worldwide.
Update: The FCC have now cleared the AWS version of the iPhone 5 which will go on sale April 12. The unit will support HSPA+ on AWS bandwidth.