For the v5.0 moniker applied to iOS 5, I can’t help but feel Apple sees this as the first major iteration of iOS — at risk of knocking all the leaps the iOS SDK took from launch to iOS 4 it certainly seems everything prior to this was just 1.x.
I can't convey how much I agree with Nik's sentiments. I've been using iOS since it was called iPhone OS 1.1, and that was on a first-generation iPod touch. And after pondering Nik's comment, it really does feel like Apple has been doing minor alterations, improvements, and polishing all this time. From what I have seen of iOS 5 via Apple's demoes and videos, and some hands-on with development devices, it really does feel like Apple took a look at everything in iOS to see what needed to be addressed.
iOS 5 makes previous versions seem like Apple was just warming up.