To crack Siri to port to other device older than iPhone 4s, it takes UDID of that phone's series. But there is a chance that Apple will close the access to UDID if this cracking thing continue to grow uncontrollable.
To refresh our memory, Siri was tested to iPhone 3GS and also to iPhone 4. It seems like everyone has been trying their hands to bring this personal assistant technology to match with their devices, and so far the jobs done well.
If you feel you are one of few who can develop what Applidum has set, visit this link to the tools to reverse-engineer Siri.