http://developer.apple.com/devcenter/ios/index.action

“iOS 5, the latest version of the world’s most advanced mobile operating system, includes over 200 new user features, and an updated SDK with over 1,500 new APIs and powerful new development tools. Start developing today to take advantage of iCloud Storage, Newsstand Kit, Core Image, GLKit, Twitter and new Game Center APIs.”

http://developer.apple.com/technologies/ios5/