AnandTech - Siri Gets Hacked: Running on iPhone 4 and iPod Touch 4G
There is speculation that Apple is limiting this to the iPhone 4S because of back of house capacity. I hope Apple ramps up their server farm to allow more devices to use the service.
Developers Steve Troughton-Smith (@stroughtonsmith) and @chpwn have managed to get Siri to run on iPhone 4 and fourth generation iPod Touch (uses the same A4 chip as iPhone 4, but has only 256MB of RAM). Even without the dual core A5 chip, Siri appears to run smoothly. Troughton-Smith said that Siri runs as fast on iPhone 4 as it does on iPhone 4S. The iPod Touch has some problems due to the inferior microphone (iPod Touch wasn't designed for calling like iPhone), requiring you to talk very loudly but the actual Siri software runs flawlessly. Below is a video of Siri running on the iPod Touch.
There is speculation that Apple is limiting this to the iPhone 4S because of back of house capacity. I hope Apple ramps up their server farm to allow more devices to use the service.