Redsn0w 0.9.3 has been released which now automatically allows you to enable tethering.
"The best thing about redsn0w 0.9.3 is that it includes this tethering hack by default. You don’t have to do anything manually as you did earlier. Instructions for jailbreaking using redsn0w 0.9.3 are exactly similar to the last release i.e. version 0.9.2 which was released last week. Follow the complete step by step guide posted here to jailbreak your iPhone and iPod touch using redsn0w 0.9.3.
NOTE: Late models of iPhone 3GS, iPod touch 2G and iPod touch 3G are all restricted to “tethered” rebooting. That means you need to connect your device to a computer to complete the boot after a reset. Also, these devices cannot have custom logos."