Interesting post about iOS5 install base as of October 17th, 2011:

The latest and greatest Apple mobile operating system, iOS 5, has been available to the public for less than a week, and yet it’s already been downloaded enough to power 33% of all eligible iOS devices. If you remove the newly-launched iPhone 4S (which ships with iOS 5), that number drops a tiny bit, but still remains an impressive 31%.

Looking at devices, we see that the iPad 2 and iPhone 4are leading the charge, with 36% and 35% iOS 5, respectively. Original iPads clock in with 33% iOS 5 proliferation, followed by iPhone 3GS (27%), iPod Touch 3rd Generation (23%), and iPod Touch 4th Generation (17%).