As of now we know that a Siri port for devices other than the iPhone 4S is a long way from release. However developers are hard at work trying to come up with a solution to bring Siri to other iOS devices. Another developer has been able to get Siri working on iPad 1.
The problems developers are facing right now is making Siri contact Apple's servers. It may take a while before developers are able to make any device other than iPhone 4S contact Siri's servers at Apple and enable full functionality for it.
Developer @Jackophone who ported Siri to iPad 1 has been working hard on this project. However it remains to be seen whether the hardware on devices other than iPhone 4S can provide the necessary power required for flawless working of Siri.
We'll keep updating more on this as it comes in.
Update : Siri connected to Apple's Servers successfully - Here's iPhone 4 and iPod Touch 4G Video - Click Here