iOS 4 device owners, rejoice! iOS 4.2 is live and ready for public consumption. Give your device a little time; the download is close to 625MB so it'll take a bit to get it going.
iOS 4.2 is available for the iPad, iPhones 3G and later, and iPod Touch devices 2nd generation and above. The biggest amount of excitement is over iPad multitasking being unleashed, but app folder enhancements as well as the addition of the Game Center, AirPlay, and AirPrint make this a must download as well.