http://www.google.com/search?q=music+genre+classification turns up some interesting results. The first link that shows up is a PDF of some Stanford researchers' attempts to have a computer classify classical, rock, and jazz music. Neat stuff! If you come across some other good articles, please post links to them.
I didn't see any links to available software that can do this classification, so the technology appears to be in the R&D stage at this point. I didn't look too hard, though. (I just got my
Principles of Digital Audio book from the library, so I've got all the reading I need right now!)