I just read this on 9to5mac, so I still need to research this a bit more, but once I figure out how to install Siri on my iphone 4, I’m on it!  And will report back.  Now its pretty much a given you will need to be jailbroken to run this, and the first few ports will be pretty slow because of some binaries needed for acceleration that are missing.  Still, if it can work just a few hairs better then what I saw in the video, we have win.

Developer Steven Troughton-Smith has been working with 9to5Mac exclusively to port the iPhone 4S Siri voice command system to the iPhone 4. Troughton-Smith was able to get the beginning steps of a full port rolling after installing the iPhone 4S Siri and Springboard files onto an iPhone 4. As you can see in the video, Siri’s interface loads up with all the features from the iPhone 4S implementation.

http://www.youtube.com/watch?v=Pue5Yl3WrZA&feature=player_embedded