Version info on wikipedia:

http://en.wikipedia.org/wiki/IOS_version_history

Market share articles:

• http://www.cocoanetics.com/2011/08/ios-versions-in-the-wild/
• http://www.marco.org/2011/08/13/instapaper-ios-device-and-version-stats-update
• http://www.quora.com/What-proportion-of-all-iPhone-owners-use-iOS4-%2a-today
• http://insights.chitika.com/2011/61-of-ipads-already-running-ios-4-january-ios-and-android-os-breakdown/
• http://www.businesswire.com/news/home/20110302005489/en/iPad-2-iOS-4.3-Coming-20-iPads …