From DevTeam:
If you have an iPhone 3GS with older bootrom and used RedSn0w 0.9.9b9, you can simply run this new version of RedSn0w on top of it
iPhone3GS owners in our comments section below noticed a problem with the untethered jailbreak available for the old-bootrom 3GS. Weve corrected that problem in 0.9.9b9d. If you have an old-bootrom 3GS and have already used last nights 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 its already installed).