Correct me if I am totally "off-base" with this anyone, but GPS for navigation for cellular devices is not totally reliant upon AT&T signal. I use "CoPilot NA" on my iPhone and I know that it works in some VERY rural places where I know for a fact AT&T's service is beyond weak!
The way I understand it is that true GPS units (ie. Garmin) use Satellite location. Just like Satellite Radio does (Sirius/XM) Cellular devices use cell towers for location and not just that particular Carriers cell towers (ie. AT&T phone can pick up VZ or Tmo tower for GPS location). Again from my understanding and experience that is how most GPS Apps work on Cellular devices.
So a GPS app should work well on the iPad for navigation (3G iPad version of course)