I've had these issues with ATT as well. In September they "discovered" my Touch HD on their network and zapped it from working. I was on the "pay as you go" plan. I switched the SIM to my old HTC Trinity and my service was useable again. Since the Trinity was one of the first full featured phones, they must not have had it in their "kill" database.
I dumped ATT like a hot rock, then went to T-mobile and I've never looked back. ATT has nothing over T-mobile.
Nexus 7 OTG USB adapter, 32gb micro SD
Galaxy Nexus GSM, Jawbone Era BT
Asus Transformer + Dock, 32GB SD, 32GB micro SD
Nexus One 16gb 10x microSD