Apple will be resetting all backups data for iCloud on Thursday September 22nd, 2011. This information comes from a post posted on the Apple Developer board for iOS and Mac, and later on reported by 9to5mac:
On Thursday, September 22, the iCloud Backup data will be reset. Backing up to iCloud or restoring from an iCloud backup will be unavailable from 9 AM PDT – 5 PM PDT. If you attempt a backup or restore during this time, you will receive an alert that the backup or restore was not successful. After this reset, you will be unable to restore from any backup created prior to September 22. A full backup will happen automatically the next time your device backs up to iCloud.
This could mean Apple is preparing to release the GM version of iOS 5. The iPhone 5 is rumored to be released and announced during the month of October.