According to TechCrunch, iOS 5 won't be released until the fall and will most likely coincide with the release of iPad 3. So if that's the case, then we could see iOS 5 released as early as September 11'.
Here's more from TechCrunch:
■ iOS 5 will launch in the fall and will be a major revamp of the OS.
■ It could well be previewed at WWDC, it just won’t be released then.
■The iOS 5 launch is also likely to coincide with the release of a new type of iPad.