According to a posting on the Apple Developer boards, the company will delete all iCloud Backup data on September 22 as a preparation for the launch of iCloud and iOS 5 .
The service (of iCloud) will be unavailable for a period of time on that day:
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.
Stay tuned for the public iCloud release.