The iPhone 3GS has been hacked to run Siri. Officially, Siri only works on the iPhone 4S but with some hacking a couple of developers managed to port Siri to the iPhone 4 and iPod touch and have now successfully got it working on the iPhone 3GS.
Just like the iPhone 4 and iPod touch version, the 3GS version of Siri is also successfully authenticating with the Apple servers which means that it can also respond to your voice rather than just show the interface and do nothing.
Siri appears to run smoothly on the 3GS other than some slight jitter when the business card slides up (could be the camera capturing that though).
It isn't clear if Apple will push Siri out to more devices in the coming months, but we suspect the iPhone 4 and iPad 2 will both get Siri eventually. As for the 3GS, Apple might still deem it a little too old to run Siri.
Take a look at the video to see how it runs on the 3GS.