Quote (MuscleNerd):
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).