And they reported the errors to iPhone dev-team which made those gang of nerd responded immediately with the new release, here's the quotation from them:
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).
And if you are experiencing the error on Redsn0w 0.9.9b9b jailbreak, you can upgrade to the 0.9.9b9d to fix errors with old-bootrom 3GS and don't worry you won't lose anything.
Here is the download links of Redsn0w 0.9.9b9d based on your operating systems:
Download RedSn0w 0.9.9b9d Windows
Download RedSn0w 0.9.9b9d Mac OS X
Afterward you can continue to jailbreak iOS 5.0.1 For iPhone, iPad, iPod With Redsn0w 0.9.9b9b For Mac And Windows.
Update: Download Redsn0w 0.9.10b1 to Untethered Jailbreak A4 5.0.1 Firmware.