Hmm, I didn't know that about the Great Prophet slingshot.
@Palpster, I don't know if this helps. THere probably is a thread where someone carefully reported the orders, but I don't know it. Generally, though, a great person will research the next available tech in his particular specialty.
So a great prophet will research Code of Laws if that's the next religious tech. A great artist will research music. A great scientist will research chemistry.
Notice in theimmortal1's post, he points out that his prophet will give you CS only if you don't know masonry. This is because if you know masonry, you'd have access to research monotheism. Monotheism, being cheaper than CS, would be the "next" religious tech.
So how do you know which techs are which categories? Some are obvious, but the XML files will let you know about those that aren't so easy.