I'm facing the same problem, and it was causing a huge battery drain/overheating problems on my 4S after the upgrade to iOS6. (I also got a data overage message from AT&T.) Additionally, I noticed some calendar entries on my phone were disappearing.
My phone and desktop (Mac Mini) are my primary calendars, synced through iCloud. I also have my Outlook calendar synced in iCloud via Google Calendar.
After reading multiple posts here and elsewhere, I tried the following:
- hard resets
- deleting and restoring my iCloud account on my phone
- restoring factory settings
- restoring from a backup
Although the battery drain problem did improve slightly, none of these resolved the problem entirely. Ultimately, I turned off the iCloud calendar sync, and replaced the calendar on my phone with that on my computer via a sync on iTunes. (My iCal calendar is correct.) This has resolved the battery drain situation, and the overheating.
From reading other posts on related issues, my educated guess is that somehow the calendar file on my phone became corrupted during the upgrade. Unfortunately, I don't think I thave access to a backup prior to the iOS 6 upgrade, so a restore from backup won't resolve the problem. (I'll have to do some detective work this weekend.)
If my guess is corrected, the only resolution is to do a restore as a new iPhone. Either that, or hope and wait for a software fix from Apple. 😕