iPhone Dev Team has released a new version of Redsn0w to jailbreak iOS 4.2.1 for iPhone 4, iPhone 3GS, iPhone 3G, iPod Touch 4G, iPod Touch 3G, iPad and Apple TV 2G.
Redsn0w 0.9.6b4 makes use of Geohot’s USB bootrom exploit used in limera1n for iOS 4.1.
Redsn0w 0.9.6b4 is available for both Mac and Windows users.