While we all wait for an untethered jailbreak solution, the Dev-Team has updated redsn0w with a "one-click" option for the tethered 4.2.1 jailbreak. Rather than the multiple screens you normally see, you can now use redsn0w on the command line to "Just boot tethered for now."
There are currently two major efforts towards an untethered jailbreak. The "jailbreak monte" backup plan is still in beta, and is only available to developers who have access to the 4.2 beta 3 build. While that is underway, @comex is working on the 4.2.1 untether and making good progress (even if slower than many of us would like). So since we're kind of stuck with the current stet of affairs for the immediate future, the Dev-Team added some command-line options to simplify the process.
You need to just specify redsn0w by location and run the appropriate arguments for your setup:
-j to ask redsn0w to "Just boot now"
-i [filename] to specify your reference IPSW
-o for old-bootrom iPod touch 2G and iPhone 3GS
-b [filename] to specify your own boot logo png
For example, if redsn0w for Mac and your IPSW are on the Desktop, you would use the following command to do a tethered boot of an iPod touch 4G jailbroken at 4.2.1:
open ~/Desktop/redsn0w.app --args -j -i ~/Desktop/iPod4,1_4.2.1_8C148_Restore.ipsw
Source: Dev-Team Blog