I ran across a CD by this Christian rock band, "Apologetix" not long ago, and they include a disclaimer which references the Roy Orbison case mentioned earlier in this thread. It seems like they pretty thoroughly researched it before they began writing "parodies". Their website states their rationale, and also has links to details of the court case, etc.
Their music isn't too bad - pretty faithful recreations of the originals songs (which are clearly credited on the cover) with heavily modified song lyrics.
Here's their FAQ on the legality of doing "parodies":
http://www.apologetix.com/faq/faq-detail.php?faq_q_id=1
David, Colorado