iOS 4.3 to iOS 5.0 API Differences
https://developer.apple.com/library/ios/#releasenotes/General/iOS50APIDiff/
iOS 4.2 to iOS 4.3 API Differences
https://developer.apple.com/library/ios/#releasenotes/General/iOS43APIDiffs/
iOS 4.1 to iOS 4.2 API Differences
https://developer.apple.com/library/ios/#releasenotes/General/iOS42APIDiffs/
iOS 4.0 to iOS 4.1 API Differences
https://developer.apple.com/library/ios/#releasenotes/General/iOS41APIDiffs/
iOS 3.2 to iOS 4.0 API Differences
https://developer.apple.com/library/ios/#releasenotes/General/iPhone40APIDiffs/