GreenPois0n should be updated to support 4.2.1 (tethered or semitethered) within a few days.
According to a recent post by the iPhone Dev-Team, an untethered jailbreak is currently available to only the iPhone3G, older iPhone3GS, and non-MC iPod touch 2G users. @comex is working hard on a method that may untether the 4.2.1 jailbreak for other devices but it will require you to have your 4.1 SHSH blobs in order to use it.
