Usual April iOS preview --> WWDC
thus by extension of that logic:
iOS 5 released at September event (first week of Sept) for existing users (remember the nightmare release that was OS 3.0?)
and
iPhone 5/4S/whatever announced at September event, released 2ish weeks later to give carriers/networks time to prepare
Only question remaining is the release of iPod Touch 5G. Do they release it on day of September event or near enough, thus before the iPhone, or will its availability be put back to coincide with the phone?
I expect the latter; can't see the Touch, the red-headed stepchild, being released before the (by comparison in Apple's eyes) prodigy that is the iPhone.