The clues have been spotted within the testing logs that software developers are getting back from Apple. Is seems like the applications that developers submit to Apple in order to get their “seal of approval” are being tested on a variety of devices running iOS 5 – iPhones, iPads, and iPods alike. Judging by the screenshot that we have included below, the iPhone 4, the iPad 2, and the third and fourth generations of the iPod touch are the most likely candidates for iOS' next major release. However, it is quite possible that older devices such as the original iPad and the iPhone 3GS may be treated to a software upgrade just like the iPhone 3G was updated to iOS 4 upon its release sporting at least some of its features.
It is speculated that the earliest iOS 5 build to be released will probably be a preliminary developer's edition for testing purposes. Understandingly, regular users will have to wait a bit longer until its official release, presumably alongside the iPhone 5's launch later this year. We will be impatiently waiting for Apple to reveal more at its WWDC event, which will take place in a little over a month's time.
source: 9to5 Mac via Redmond Pie