In a recent tweet by iH8sn0w, he has stated that iOS 5 Beta 5 will expire on September 14, 2011. This means that if you use your iDevice past that date and time, it will lock up, forcing you to restore to the GM or final version of iOS 5 (depending on when the final version is released). If haven't heard already, iOS 5 Beta 5 was released today with some new features.
iOS 5 Beta 5 Set to Expire on September 14