David Smith’s iOS version stats from his apps:
The data for 4 days shows a clear and very consistent progression. Users are updating at a rate of roughly 7.5%/day, leading to a total adoption of around 30% so far.
30% of his userbase has upgraded to the four-day-old, no-new-features, not-marketed iOS 5.1.1 release. That’s incredible.
Also, if you’re a developer, listen to his podcast.