Following previous attempts to port Siri to the iPhone 4 and iPod touch, developer Ryan Petrich, assisted by Steven Troughton-Smith and others, has successfully ported Siri to the iPhone 3GS, although its performance isn’t quite up to par.
According to 9to5Mac, the microphone performance is significantly worse than with the iPhone 4S, or even the iPhone 4, and the speed and performance aren’t quite up to the standard you might expect, but it is nevertheless coming. See the port in action in the below clip:
