You should check out this Malcolm Gladwell essay (http://gladwell.com/2004/2004_11_25_a_borrowed.html). I read it in What the Dog Saw. About 1/4 of the way down there is a bit about the Beastie Boys and Andrew Lloyd Weber and how some musicians copy their own songs and most borrow something from other artists. It talks a lot about copyright and ownership and what you can really own (a note, a beat, a riff).