The iPhone hacker p0sixninja, behind GreenPois0n jailbreak tool has just posted on his Twitter account that he will update GreenPois0n tool to support the newly released iOS 4.2.1 on all iOS devices.
GreenPois0n should be updated to support 4.2.1 (tethered or semitethered) within a few days.However, we'll keep you updated whenever the tool is available for download and we'll post full how-to guides to use the tool, stay tuned.