Chpwn and Steve Troughton-Smith have successfully ported Siri to the iPhone 3GS. Ryan Petrich has just tweeted the news on his account. And here is a video below showing Siri working on iPhone 3GS.
Siri Ported To iPhone 3GS
Before we have learned that Siri have been successfully ported to iPhone 4 and the iPad 2. But there was no news on if the iPhone 3GS can run Siri. But the video below shows that it can run Siri.