inspiretech iPhone 3G 3GS Dock to 3.5 Auxilary Aux Audio Cable
I bought this cable to connect to my car's AUX input.  I have the iPhone 3GS with 3.1 OS.  On connection I got an error saying the accessory is not compatible with the iPhone.  However, if you ignore that error message the music does appear to come through OK.  So, not great but overall not bad.