From the Dev Team blog:
The b3 version of redsn0w fixes a problem where re-running redsn0w over an existing jailbreak would cause MobileSubstrate-based apps to stop running until MS was installed again. Now you can re-run the redsn0w jailbreak step without worrying about that (but still remember to de-select the â€œInstall Cydiaâ€ option if itâ€™s already installed).
Also if RedSn0w tells you that no identifying data was found when it auto-detects your IPSW, then you can pre-select your iOS deviceâ€™s 5.0.1 IPSW in RedSnowâ€™s Extras > Select IPSW menu.
Download RedSnow 0.9.10b3 for Mac.
Download RedSnow 0.9.10b3 for Windows.