It's not expected until this fall, but Apple's already been giving the thumbs up to iOS 5-compatible apps. Mashable's app spells out "iOS 5 compatibility," while Camera+ skirts it with "compatibility with that upcoming OS That Must Not Be Named."
iOS 5's beta 4 has been available to developers since late-July, and the fact that Apple is already approving apps that support the new OS suggests that perhaps we don't have too much longer to wait. (Though obviously these app updates are based on iOS 4.3 rather than the iOS 5 SDK).
With the iPhone 5 expected to launch in either September or October (depending on which rumor you wish to believe), it wouldn't be unfeasible to hope that iOS 5 won't arrive too much later. [Cult of Mac]