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.
Basically, Apple wants you to buy the iPhone 4S, and they're not going to allow you to use Siri on any other devices if they can help it. Personally I would wait for the iPhone 5 or iPad 3. A sluggish voice assistant that searches Google for you doesn't sound very helpful to me. Plus do you really need your iPad to tell you in a woman's voice where the nearest Chipotle is located?