Geohot is back at it again and it looks like he has already found a method to jailbreak the iPhone 3GS!
Details of the exploit are still being gathered; however, it looks like GeoHot was able to insert a custom command to iBoot meaning sigchecks have been bypassed (thanks mav). This should be a start for the iPhone 3GS (N88AP) jailbreak which has been dubbed purplera1n.
Geohot has already activated purplera1n.com but is hosting nothing but an image as of now.
The really good news is that if the jailbreak is successful the UltraSn0w unlock should be compatible with the iPhone 3GS as well.
I hope soon. I have been following some of the dev team on Twitter and they are begging for money to get 3GS's for some of the devs. That was a big disappointment -- you mean they didn't all get them at 7AM on the day of release and have been working on it nonstop?? (only about 1/4 kidding).