Apple said that Siri is an iPhone 4S ONLY feature and they don’t plan on bringing it to ‘older’ devices, some developers managed to fully port it to the iPhone 3GS, iPhone 4 and iPod Touch 4. Unfortunately, they can distribute the port because of legal reasons , but somebody managed to distribute the next best thing: Siri dictation.
iPhone hackers chpwn and Steve Troughton say they’ve gotten Siri working on the iPhone 4 and fourth-gen iPod touch, and they’ve got video to prove it. So it is possible. Sadly, though, both hackers say that whatever method they used for exploit is not for public release any time soon, so the rest of us poor suckers will just have to keep twiddling our thumbs.