Now that redsn0w has been updated to jailbreak iOS 4.2.1 you may be wondering what's involved in actually jailbreaking your iPhone. Here are some tips and instructions to help. First of all, if you have an unlocked iPhone don't update your device to the official iOS 4.2.1 or use the redsn0w 0.9.6b4 jailbreak. You will lose your unlock.
UPDATE: Comex has released an easier method for newer 4.x firmwares. Click here for instructions on how to use JailbreakMe 3.0 to jailbreak your device.