Clark found that his iPhone 4 was draining the battery faster than his car dock could charge it and found data usage going from 85% to 100% of his AT&T usage overnight. His solution:
Go to your System Settings on the phone. Go to Cellular. Scroll down. It should say “Use Cellular Data for:” Apparently iOS6 defaults to “iCloud Documents on.” That means it was syncing everything on iCloud while on cellular rather than WiFi.
If you are on a metered data plan, it would be a good idea to check your cellular data usage settings in iOS 6. I believe that iCloud documents is just the actual documents and not your calendar, reminders, etc., however.
Update: I did some tests, and turning off cellular data for “iCloud Documents” does indeed only stop documents from syncing (your calendars, reminders, etc. still sync).