The GreenPois0n untethered jailbreak for iOS 4.2.1 has been released and it allows you to jailbreak any iOS 4.2.1 device completely untethered. This will work on the iPhone, iPod touch, and iPad.

How to Jailbreak iOS 4.2.1 Untethered with GreenPois0n 1.0 RC5

Backup your iPhone or iOS device before proceeding. Do not use this jailbreak if you rely on a carrier unlock!

Download GreenPois0n untethered jailbreak

Connect the iOS device to the computer with GreenPois0n 1.0 RC5

Launch GreenPois0n and click on “Jailbreak”

Follow the onscreen instructions and counter carefully, this lets you know how long to hold buttons

Press and hold the sleep button for 2 seconds

Continue holding sleep, press and hold home for 10 seconds

Release sleep button, continue holding home button

Your iOS device should now be in DFU mode and it is ready for the jailbreak

Click on “Jailbreak” and let the exploit install

After your device has been jailbroken, go to your iPhone and look for the green “Loader” icon and launch this to install Cydia

GreenPois0n 1.0 RC5 includes the ability to have custom animated boot logos. This version of GreenPois0n is superior to the latest Redsn0w release because it does not require developer saved SHSH blobs of 4.2b3.

Troubleshooting: If you are having problems getting Loader to download and install Cydia after using GreenPois0n RC5 jailbreak, the servers may be overloaded. In this case try again, or you can do the following:

Turn off your device

Download and open Redsn0w 0.9.7b6

Make sure the only option checked is “Install Cydia”

Follow the instructions in Redsn0w app and Cydia will install

Thank you to Rock_Bege and our other commenters for the solutions.