High iOS 6 Data Usage? Turn off iCloud,
For those of you with unexplained excessive data usage, I've pinpointed the problem to a bug in iCloud. Unfortunately, turning off individual iCloud services will not solve the problem, you'll have to delete the iCloud account from the phone.
Here's a video demonstrating the problem.
I hope this helps. If it solves your data usage problem, be sure to report it to Apple and spread the word.
iPhone 5, iOS 6