The long awaited untethered jailbreak for iOS 4.2.1 is here, albeit in limited fashion. The instructions below pertain to Redsn0w 0.9.7b1, a beta release of the so-called "backup plan" untethered jailbreak. This release is not the final release and, being a beta, requires a bit more effort than the eventual final release should. This jailbreak is also limited to the iPhone 4, iPad and iPod Touch 4G.
The following provides step-by-step instructions on how to jailbreak your iPhone (or other iOS device) using Redsn0w 0.9.7b1.