Connecting To An Automotive Diagnostic Computer?

An Anonymous Coward writes: "I was wondering if anyone has tried to hook up a PC to the diagnostic computer on an automobile. Each time the little idiot light on my Blazer comes on I have to cough up $20 - $40 just to find out what the problem is. This seems to me like an amusing and only moderately difficult hack - I can't believe that I am the first person to think of it. However, I can't seem to find any information on the topic at all."