iOS and Mac developer David Smith on the latest iOS 5.1 upgrade numbers based on the data from his popular Audiobooks app:
It took iOS just 15 days to get the same percentage of users on the latest OS version as are currently on any single version of Android.
It took just over two weeks to get 61% of users on iOS 5.1. This is the same distribution percentage as the most widely-used Android version, 2.3, Gingerbread — which is a year and a half old. Two major Android versions have been released since then, but combine for under 5% of total usage still.
And if you narrow it down to only include those who can get iOS OTA updates (iOS 5 users), the adoption rate of iOS 5.1 in two weeks is almost 80%.