Developer David Smith has posted upgrade stats for his users in terms of their adoption to iOS 5.1, and as you can see above, the curve is pretty steep. iOS upgrading has always been sort of an issue for Apple -- in the past, users (I mean outside of the elite readers and staff of TUAW, of course) haven't been quite so willing to upgrade so quickly, and Apple is always trying to push users towards the newest iOS versions for developers' sake. But clearly iOS 5.1 has made a big splash, especially since it's the first big update that's available to download right over the air onto your device, rather than requiring a link up to iTunes on a computer.
And outside of the Apple ecosystem, this is still pretty phenomenal: Smith says that in 15 days, the latest version of iOS has just as many users as any single version of Android. That's the fragmentation that developers are trying to avoid with iOS, and that Android is suffering from. The more you can unite your users onto the same platform and operating system, the better chance you have of supporting them all with whatever resources you have.