I know you all love Slashdot, but here is their take on Siri:
According to the article linked, Siri doesn't actually do much, it just works the following way:
1. User says "Do I have new mail?"
2. Siri encodes a sound file
3. Siri sends stuff to Apple
4. Apple responds back with carefully researched answer.
5. Siri receives the answer.
6. Siri says "No, you don't have any mail"
The hard part is done on Apple server. I bet if you could access the broadcom chip, you could encode stuff yourself without consuming CPU power. But Apple might be using a supercomputer or equivalent for Siri Artificial Intelligence, so I doubt R-PI chip can ever reach such quality and responsiveness. But hey, don't give up - give it a try
And I did check links Asmodeus posted, I understand Simon-Listens is powered by Julius? I could get that from repositories, so I am going with that.