It’s really bound to happen for every new version of iOS 4.2.1 and the Chronic Dev team is the first one to release it (albeit with some limitations).
Biggest limitation as of now is that it’s only available on the Mac (Windows and Linux might follow soon).
This is just a jailbreak for 4.2.1 but there’s no SIM unlock yet. Feel free to try it with your iPod Touch, iPad and iPhone 4.
See Greenpois0n here.