The Iphone Dev Team has released a partial jailbreak for iOS4.2.1 but it's not for the faint-hearted.
The update to the redsn0w tool comes with a list of caveats as long as your arm and is recommended for use only by jailbreak developers who want to get their Cydia Apps ready for the official Apple release of the much-delayed of iOS4.2.1.
As the public versions of Cydia are not yet ready for release, the latest redsn0w allows custom data bundles of up to 15MB to be upload to iDevices without the public user interface.
The warning that redsn0w 0.9.6b3 should only be used by tinkerers capable of compiling their own custom version of Cydia should be enough to keep most casual users away, but just in case The Dev Team says: "PLEASE CONSIDER THIS AN ADVANCED TOPIC!!
It’s not meant for the masses because it involves rather nerdy things like command lines and tar files. But for those who know how to use this new redsn0w feature, have fun!"
If you don't know how to use it, you will almost certainly lose your carrier unlock, and chances are you'll end up with a very expensive paperweight.
The official release of iOS4.2 is now expected by many to show its face on Wednesday and if recent history is anything to go by, a public jailbreak will follow hot on its heels.