iPad 3 used up 250 MB of LTE data in 3 hours without active usage
If you use Photostream or iTunesMatch there is a good chance that one of the two uses cellular data unexpectedly. As far as I remember there was a problem (before iOS 6) with syncing these two services: when you started the sync in WiFi mode and then switched to cellular (e.g. because of leaving from home), the sync would continue independent of the setting disallowing usage of cellular.