So you want to get Siri on your non iPhone 4S Device?
Good news!
(If you have an iPhone 4, 3GS or iPod touch 3rd or 4th generation)
An App has been released on Cydia (the main directory of applications for jailbroken iDevices)
called Spire.
It gives you Siri functionality with two small drawbacks, you must have an iPhone 4, 3GS or iPod touch 3rd or 4th generation, and you also must have a Siriproxy server configured with your device.
The way Siri works is it records your voice, sends the recording to Apple's servers, which then decode your voice into a command and the servers send back the command to the iPhone 4S which then carries out the command.
However, some key pieces of information are needed for Apple to send back the command, called iPhone 4S authentication tokens, which can only be taken from an iPhone 4S. There are 4 tokens sent, 1 of which changes every 24 hours.
What a Siriproxy server does, is it sends the authentication tokens along with your voice recording to Apple for translation. There are many Siri proxy servers that you can find with a quick internet search, but most are overloaded within minutes of their posting. You can pay for certain Siriproxy servers IF you can find the right one. I have provided a list of servers at the bottom, but the best way to have a reliable Siriproxy is to make one yourself. I have provided a YouTube video near the bottom explaining how to do this.

I take no responsibility for damage inflicted upon your device from instructions on this blog, but if damage does occur, please tell me how you managed to do it.