iH8sn0w, infamous developer behind Sn0wbreeze, iReb and F0recast, managed a tethered iOS 4 jailbreak for iPhone 3GS with new bootrom. Needless to mention that tethered jailbreak means that the jailbreak needs to be reapplied every time you power down your iPhone. If you let the battery run to zero or restart your iPhone then you will need to reapply the jailbreak procedure once again.
You also need to have your 3.1.2 SHSH saved on file, procedure is very long and chances of failure are very high even for advanced users. If ...