According to a German Web site claiming to have a reliable source, iOS 5 and the new-look MobileMe, widely expected to become a free syncing service offered by Apple, will be unveiled at an event in early April to be held on the Apple campus in Cupertino, Calif.
MacRumors.com, via Macerkopf.de, is claiming to have knowledge of the event, to be held "on Apple's campus in the 14th or 15th week of the year."
This rumor may be closer to an educated guess, though, as Apple announced (and showed a preview of) iOS 4 on April 8 last year. It is likely that Apple's development schedule for iOS, iPhones, and iPads remains fairly stable, making an early April preview of iOS 5 somewhat probable.
In the coming weeks, expect to see more leaks and rumors regarding the content of iOS 5, especially new features that relate to possibly upgraded hardware in the iPhone 5, expected to be released sometime this summer.