Apple has released iOS 4 (new name for iPhone OS 4) for iPhone 3G, iPhone 3GS, iPod touch (second generation and later).
iOS 4 brings over 100 new features, including multitasking, folders, Mail improvements, iBook support and more. The operating system is compatible with iPhone 3G, 3GS, and 4, as well as second- and third-generation iPod touch models. Not all features are compatible with all devices, however, with features such as multitasking, custom wallpaper, and Bluetooth keyboard support requiring a minimum of an iPhone 3GS or third-generation iPod touch.
And upgrading to iOS 4 may take a while...
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