Here's an interesting take on this issue from the New York Times:

http://www.nytimes.com/2003/07/12/arts/music/12DYLA.html?th

One obvious point the writer makes is that lifting several lines from a work of prose and putting them into songs is not the same level of plagerism as stealing words and/or music from one song for another purportedly original song. Also, there are some interesting observations about audio "sampling."

You have to sign up as, or already be, a "member" of the NYT site. I believe there is a username and password for Mudcatters, but I didn't remember it and signed up as myself. Anyone have that info?