I'd love to build this data into a neural network and see if I can come up with robosongs. It should be quite good at coming up with "What chord should come next?"
I'd love to hear a Markov chain. I don't think it'd be pleasant. But it'd be fun.
MC trained on classical composers: http://www.youtube.com/watch?v=qOZ2Q-Ls48U
Iannis Xenakis was a major 20th century composer who incorporated MCs into some works: http://www.youtube.com/watch?v=DTzOWKaDrVI
Haven't seen a serious effort with pop/rock though. Lyrics might be the sticking point.