iPhone3GS owners in our comments section below noticed a problem with the untethered jailbreak available for the old-bootrom 3GS. We've corrected that problem in 0.9.9b9d. If you have an old-bootrom 3GS and have already used last night's redsn0w on it, you can re-run it again without losing anything. Just use this new version, go to Extras->IPSW and manually select the 5.0.1 IPSW, then go back and Jailbreak it again (but you can uncheck Cydia because it's already installed).
You can download the latest version of RedSn0w from here: Mac, Windows.