From a tweet by MuscleNerd, it looks like GeoHot may surprise us once again because he has an untethered jailbreak exploit for iOS 4.2.1. Before that GeoHot made a surprise come back and released Limera1n jailbreak for iOS 4.1.
ipodguy79 Tweeted: @sherif_hashim @MuscleNerd @iphone_dev @chpwn keep up with your soap opera and geo will be slapping another reality check on you
MuscleNerd responded: I hear geohot does have an untether actually! Though not for all devices.