@nanotube
That's a great analysis -- helpful information. Thanks for sharing!

I never used a calling card though; however, I still double-checked all the calls I made (which weren't much at all) and all the numbers are familiar to me.

I'm really convinced that it's a network and/or OS issue. Even if I used data while not in Wi-Fi, there's no way I could have used huge chunks of data unless there were transmissions that I never initiated. As I've mentioned, I never changed anything but updated to iOS 5.

I've been checking other forums as well but it doesn't seem like it has been resolved or that other users have heard anything concrete from either AT&T or Apple (unfortunately).