Are you sure? I thought they prorated the data as well. I've used AT&T overseas a number of times and I thought I ran into some roaming because I shut off my plan too early in my cycle.
I know this works for the phone. But I'm not sure this works on the iPad. I've gotten a message from AT&T that they couldn't prorate when I've gone to shut of iPad data. Could just have been a bug at the time. I was working on three different iDevices.
Worth double checking if you use up most of your data.
Cheers.