The Chronic Dev-Team has finally unleashed greenpois0n RC5 for Mac. The jailbreak is untethered, and works on all devices without the need for those silly SHSH blobs. A Windows version is in the works, and should be released shortly. Also, it should be noted that the download could become somewhat slow (and the Greenpois0n website could even crash for a little while). Fear not: that’s simply the result of a vast number of people trying to access the program at once. In other words, be patient!
The greenpois0n exploit has been patched in iOS 4.3, so make sure to hold off on upgrading when Apple pushes the public release, which could be in the next couple of weeks.
So, what are you waiting for? Hit the source link, get to downloading, and jailbreak your iOS 4.2.1-running device!
Update: ih8sn0w has a mirror [direct download] up for the jailbreak, which you can use if the official download isn’t working for you. Remember, it’s still for Mac OS X only!
Update 2: For those of you having problems (which seems to be nearly everyone at this point), p0sixninja is on it. Apparently there are some issues with decompression, which will hopefully be worked out ASAP. You can view the tweet here.
Update 4: Greenpois0n has been updated to RC5_2, which fixes the “greenpois0n initialized” bug. If you’re still having problems, use this new version. Available on the GP homepage.