Gonna compile all the latest happening of the iOS 4.2.1 UNTETHERED and TETHERED jailbreak here.
Lets take a look at the different tools that allows you to jailbreak into TETHERED state first:
Firstly, there’s the redsnow tool, which allows you to jailbreak into tethered state using 4.2.1 firmware files. This applies to all devices.
For untethered, there’s quite a number of tools for it:
Idevice 2G,3G and 3GS older bootrom: redsnow
For newer devices like iphone 4, ipad: theres currently a beta version of redsnow on the works which uses 4.2 developer built 4.2b3 SHSH. So if you got your handset brand new at iOS 4.2, tough luck. You cant jailbreak using redsnow. You will have to wait for @comex jailbreak.
For those waiting for @comex jailbreak, there are rumors that he will only release his jailbreak when iOS 4.3 is out.
So once again, we have to wait.