To avoid this problem Apple has added a button in the iPad to let you check your usage. This link does not keep track but takes you directly to AT&T’s web site and queries your actual account. This means that the usage is tied to the SIM and not the iPad. The counter even survives a full restore of the iPad. You can see your currant usage and your billing period. Should you go over, you can immediately upgrade to the unlimited plan and have that action back dated to the beginning of the period. There’s even an option to add a International Plan. I’ll have to research that option.
We’ve all heard the stories (or in my case experienced it first hand) of people going overseas and using the data usage meter in their iPhone to make sure they have not gone over only to find out that the meter in the phone lies. You see the problem is that the meter in the phone records actual bits send and received while the phone company uses its own definition where the smallest unit of measurement is 1 kilobyte. So if you download 10 bytes of data you are charges for 1024 bytes. At least that’s what I can glean by looking at my bill and comparing it to the phone.