iH8Sn0w reports that iOS 5 Beta 6 will expire on September 29, 2011.
iOS 5 beta 6 expires on: Thursday September 29, 2011 @ 8:00:01 PM (Eastern Standard Time)
It's unlikely that this date offers any insight into the official release date; however, it's important to remember to upgrade your device before the firmware expires otherwise you could lose access to your phone at a very inopportune time.
Meaningless date, beta 7 and/or GM will be out before then anyway. They set the expiry date far out in the future to ensure devs have plenty of time to update to the most recent beta/GM.