Well, I just started an open source project to gather the data for such systems. It is starting as a database/graph of symptoms and conditions. But the vision is bigger than that.
I've been doing a lot of work lately with automated Abductive Inference, and medical diagnosis has been cited as one of the key uses of same. Perhaps there could be some opportunity to collaborate (everything I do is open source as well, btw). If you guys would like to talk, feel free to hit me up at prhodes@fogbeam.com