The jailbreak developer, iH8sn0w has updated Sn0wBreeze to perform an untethered jailbreak of the old bootrom iPhone 3GS on iOS 5.0b1.
Snowbreeze is basically the Windows counterpart of PwnageTool. Like PwnageTool, it allows users to create a custom IPSW for your iDevice which jailbreaks your device but doesn’t update the baseband.
This jailbreak is intended for developers only and can be download from here.
Source: iH8Sn0w