Chpwn and stroughtonsmith managed to get a fully working Siri port on the iPhone 4 and iPod Touch, but have not publicly released instructions documenting the process. Due to legal reasons, the duo have decided that until they are able to extract the needed files from the 4S legally, they will not be sharing what they know. You may not like it, but if it is going to be done, it’s better to wait for it to be done right.
Anyway, I hope that clears up some of the technical and legal reasons why distributing a build (or instructions) to run Siri on older hardware isn’t possible at the moment. When we have the ability to decrypt the encrypted iPhone 4S firmware file — to extract the Siri files legally, without the need for an iPhone 4S — and we have an iPhone 4S jailbreak to obtain the other nececssary information at a mass scale, hopefully this can become a reality and everyone can try out Siri on their older devices. Until then, showing you a video that it is possible is the best we can do.
Hit the source for the in depth explanation.