It's already been proven Siri can be ported to an iPhone 4. Meaning there is no technical reason for it to be exclusive to the iPhone 4S. This also means it should work with the iPad or iPad 2, but is it really worth it?
A hacker known as Jackoplane did manage to add the voice assistant to a jailbroken iPad, but just like the iPhone 4, it isn't set up to communicate with the Apple servers. According to the jailbreaking community, any iOS device should be able to disguise itself as an iPhone 4S, and trick the servers into communicating with it. The voice assistant could also be set up to query Google, but as of right now it's pretty useless.