This is a hot topic on another forum. It is $20/month, unlimited data. The firmware in the Mobley has been modified to try and make sure you are actually using it in a moving vehicle. Some of the AC adapters work and some don't. The educated guess is that the voltage supplied has something to do with how they determine if you are in a vehicle.
This might violate the "terms and conditions", but some people have simply taken the SIM card out of the Mobley and inserted it into another "hot spot" that is AT&T compatible and the problem is solved.