Ok, for those of you that are trying to use Greenpois0n to jailbreak 4.2.1 and getting the "Failed" error, here's what I did and it worked. NOTE: The steps below assume that Greenpois0n.app is on your Desktop. 1. Make sure you're using Greenpois0n RC5_2 2. Make sure device is connected to your Mac 3. Open Terminal 4. Type "cd Desktop" without quotes and hit enter 5. Type "open Greenpois0n.app/Contents/MacOS/greenpois0n" without quotes and hit enter. A second Terminal window will open and Greenpois0n will start 6. Follow the instructions in Greenpois0n to enter DFU mode and it will jailbreak 7. Once jailbroken, a green "Loader" will be added to your device. This is supposed to allow you to install Cydia, but mine would not. If yours is the same, go to step 8 8. Turn off your device 9. Open Redsn0w 0.9.7b6 10. Make sure the only option checked is "Install Cydia" 11. Follow the instructions in Redsnow 12. Voila! Cydia is installed, and you have an untethered 4.2.1 jailbroken device!