A post appeared on the Apple Developers Forum this afternoon which indicates that Apple is planning a reset of iCloud Backup data on Thursday, September 22nd. This reset may prepare Apple for the release of iOS 5 GM which should be released around the same time of the reset of iCloud. Once iOS 5 GM releases it will be the last before iOS 5 officially releases in October. The reset of iCloud data will not affect data from mail, contacts, calendars, reminders, or bookmarks.
iOS 5 should release about the same time as the iPhone 4s or iPhone 5 releases in early October which current predictions state.