Excethe for the original iPhone release, major iOS announcements were done at the WWDC around June with a final release in September. I this related to the fact that major releases comes with both new features, usability updates and new APIs. The latter is the one requiring developers lengthy beta phase for testing Apple’s new stuff. I wish Apple could separate releases this way: updated features with no API changes releases and augmented APIs releases. New APIs could be introduced at a much lower pace as iOS is a pretty much mature operating system.