greenpois0n is a tool for Mac and Windows that jailbreaks iOS versions 4.1 and 4.2.1 (on iPad 1, iPhone 3GS and 4, and iPod touch 2nd generation, 3rd generation, and 4th generation).
greenpois0n runs on already upgraded un-jailbroken devices.
Steps: Download greenpois0n for Mac or Windows. Plug in your device, run greenpois0n, and let the jailbreak complete. On the device, use the new Loader app to install Cydia.
Note: if Loader fails to install Cydia, run redsn0w with only "Install Cydia" checked.
Note for unlockers: if you upgrade iOS using iTunes, it will upgrade your iPhone's baseband, which negatively affects iPhones unofficially unlocked with ultrasn0w. Instead use redsn0w's "Custom IPSW" feature to upgrade and jailbreak your device while preserving your baseband.