I hope that iOS 5's OTA updates we could get to the point where iOS versions older than a few months are considered obsolete.
Apple has done a great job at fostering this culture of upgrading as soon as possible. Microsoft and Google still have problems were the average consumers see OS updates as something that can cause problems, not bring new features.
While a very nice idea, I'm not convinced it much will change due to old hardware being incompatible with new versions of iOS. It's rarely the x.y versions which are the barriers to progress, but the x.0 versions.
When the iPhone 5 comes out this fall I think by June of next year there will be enough penetration of iOS 5 that some apps will be able to put that requirement on it.
Heck if we're lucky iOS 6 will finally kill support for non-Retina displays.