Here is a post from the AT&T Forums indicating that this may indeed be an issue with AT&T recognizing the new nano-SIMs as this user claims to have his iPhone 5 working on the Microcell but using a cut-down iPhone 4 SIM.
Also, my iPhone 5 is recognized as a "flip-phone" icon with "iPhone 2" label showing on the AT&T user settings homepage - further indicating to me that AT&T is not recognizing iPhone 5 appropriately and this may be part of the Microcell issue we are experiencing.
From AT&T Forums:
"Gentleman, I dont belive this is a phone issue but more of a nano sim problem, I ordered my iPhone 5 using another lines upgrade, since the sim that came in the iPhone 5 was for the wrong number, I decided to cut my iPhone 4S's sim down to nano size and my iPhone 5 is working as it should on the Micro cell while my Brothers is not and he's using the sim that came with the iPhone 5. just my two cents..."
Also, my iPhone 5 is recognized as a "flip-phone" icon with "iPhone 2" label showing on the AT&T user settings homepage - further indicating to me that AT&T is not recognizing iPhone 5 appropriately and this may be part of the Microcell issue we are experiencing.
From AT&T Forums:
"Gentleman, I dont belive this is a phone issue but more of a nano sim problem, I ordered my iPhone 5 using another lines upgrade, since the sim that came in the iPhone 5 was for the wrong number, I decided to cut my iPhone 4S's sim down to nano size and my iPhone 5 is working as it should on the Micro cell while my Brothers is not and he's using the sim that came with the iPhone 5. just my two cents..."