According to 9to5Mac, Apple has begun widespread testing of iOS 5. The new OS is being tested on all manner of devices, including iPhones, iPads, and iPod Touches.
This evidence comes from the above screenshot showing OS versions used to access websites. This is most intriguing, since widespread testing usually indicates that a release is due within a few months. Could this mean iOS 5 will debut at WWDC? Time will tell.