Greenpois0n RC5, the latest Jailbreak for iOS 4.2.1 iPhone, iPod touch, and iPad, has been updated to version 2, which should fix some of the issues users have been experiencing. Mainly the issue where your jailbreak will just sit at "Greenpois0n Initialized". Hopefully this version will take care of that bug. I know a few people have been having issues even loading Greenpois0n.com. There are also several mirrors available, so keep checking the comments for the latest on those.
Also, some users have been noting that their Cydia icon is completely missing or they can't get Cydia to install. You can alternately use redsn0w RC8 to install Cydia. Basically use Greenpois0n to jailbreak untethered and then run redsn0w just to install Cydia.
Let us know in the comments if the second version of Greenpois0n is working any better for you! If you need instructions on jailbreaking under iOS 4.2.1, check out our how to jailbreak via Greenpois0n walkthrough and if you need more help, our Jailbreak Forum.