George Hotz, the one who came out with iPhone 3G unlock had stated that the "24kpwn" exploit used for of iPod Touch 2G jailbreak will work for iPhone 3GS. Hotz at the age of 17 had broken through the most popular iphone allowing it to be used by many other cell phone networks. Follow his blog for more infomation.
But please Beware software has still not been released. This is what development team of iphone has to say in their blog
" Unfortunately, there are predators out there that are counting on your over-exuberance. Maybe we should call it yell0w fever. One very recent example is a certain yellowsn0w221 page on wordpress.com. Do not download anything from that page if you’re on a PC, else you’ll be infected with a virus. The page talks and talks about a supposed Firmware 2.2.1 yellowsn0w exploit, but it’s all a ruse to get you to download and infect your PC."