Firmware updates are always alarming for a jailbreaking enthusiast—who knows if it'll ruin all the hard work that's gone into the unlocked device? But iPhone users don't have to worry, since 3.0.1 is no less hackable than 3.0.
The iPhone Dev Team put up a post detailing instructions on how to use the current versions of redsn0w and ultrasn0w on 3.0.1, and it's pretty easy: Jailbreaking with redsn0w only requires you select the IPSW file for OS 3.0, and then you can use Cydia to install ultrasn0w and everything will be perfect in the land of semi-approved iPhone tinkering. Phew! [iPhone Dev Blog]