Apple has announced that it intends to reset developer iCloud Backup data on Thursday, September 22nd. The announcement was made on the Apple Developer Boards. The reason may be that Apple is getting ready to release the iOS 5 GM.
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.
GM stands for Gold Master and should be the final release of the iOS 5 software before it is made publically available. This could be as early as the first week of October along with the iPhone 5 and iPhone 4s.