go on iPhoneFix.de V2oo9.2 - Largest iPhone related filebase on .NET! No.1 File Source! to the download section, and, download the file under the ultrasn0w folder (windows only).
I know its only made for the 3G, but it might work for the 3gs. If not, just uninstall.........

good luck, I havent tried it because I dont own a 3gs.............