"There appears to be a system-wide problem with the AV Foundation framework in iOS 6.0.0, resulting in significantly higher data costs to iPhone users and internet distributors. Users who have not done so should immediately upgrade iOS 6.0.0 devices to iOS 6.0.1, which we can confirm appears to fix the issue on Wifi."
Hmm. It appears that this data bug is even worse than I thought, causing as many headaches for content providers as it does for users. Maybe I should finally get around to upgrading.
On the other hand, BGR suggests that this hasn't been entirely fixed in iOS 6.0.1.