iOS 4 : Software Update

iOS 4 software update the newest version of the world’s most advanced mobile operating system, which includes over 100 new features and 1500 new APIs for developers.

iOS 4 works with iPhone 4, iPhone 3GS, and iPhone 3G. Not all features are compatible with all devices. For example, multitasking is available only with iPhone 4 and iPhone 3GS.
Read more : http://www.apple.com/iphone/softwareupdate/