I just got the new iPad. I had an iPad 2 with almost all of the 3gb data left on the plan. When I set up the new iPad with the backup from the iPad 2, the data transferred correctly. I checked the usage and it was exactly what I had left on the iPad 2. Then I restored the iPad 2 and set up another data account on it. Gave the iPad 2 to my father to start using, everything seemed fine.
Thirty minutes later, picked up my new iPad to start using it, a notice pops up that I don't have any data left. I was on ATT 4G at the time. I hadn't even used it since setting up the new iPad and checking to see that my data transferred. I called ATT customer service, and while I was talking to them, they restored my data to were it should have been. No explanation about what happened and it has been fine since.