Preview at WWDC likely in June, followed by September launch.
Apple to Reset iCloud Backups on September 22nd, Ahead of iOS 5 GM?
Apple is planning to reset all iCloud backup data on September 22nd, 2011. As noted by 9to5Mac, the announcement was posted to Apple's developer site for both iOS and Mac developers:
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.Apple's iOS 5 has been undergoing developer testing since its announcement back in June. Developers have been able to test iCloud for backups and storage during the developer betas.
Apple has provided 7 betas to developers thus far, and had been previously rumored to be offering the Golden Master (final version before release) on September 23rd. That same rumor had also incorrectly pinpointed a Beta 8 release a few days ago, but the 23rd date seems to line up nicely with this iCloud backup reset.
Apple had said that iOS 5 would launch "this fall" which technically begins on September 23rd.