The Verge has compiled all of the locations that offer next-generation LTE service amongst the three major US carriers that offer the iPhone 5. To keep things short, it seems that, as of right now, your best bet for LTE coverage is with Verizon. AT&T seems to be scrambling to build out their LTE service, and my carrier of choice, Sprint, is dead last in this regard.
If LTE service is a determining factor for your upgrade to the iPhone 5, you might want to hold out unless your a Verizon customer, or aren’t afraid to change carriers. This will be a non-issue with time, but for now, is kinda embarrassing for AT&T and Sprint.