After I upgraded to my Iphone 5 I noticed it would not associate with my Att 3G Microcell. I tried all the standard stuff - re-booting the cell, etc., no joy. Also noted that my other iPhones (4 and 4s) had no issues with the microcell.
Called ATT Microcell support . Apparently they have had a bunch of calls on this. They told me Cisco and Apple are working on a Solution.
I then looked on the apple support page
Some crafty fanbois out there figured out to remove the microcell from your account and go thru the painful process of adding it back in.
An hour later, when it came back up - I had 5 bars of microcell joy.
Apparently ATT is aware of the issue but not the solution....--
Nothin' left to do but smile smile smile