iH8Sn0w has successfully performed a tethered jailbreak for iPhone 3GS with new firmware  iOS 4 and the new bootrom. Unfortunately this jailbreak requires users to have 3.1.2 SHSH blobs saved.

Alright... If anyone has their 3.1.2 SHSH blobs, NOT 3.1.3. Then I'll see if I can produce a tool that will do this job...

Here is a video demo posted by AKinGdom1:

The utility will be available soon. Might be as soon as tomorrow.