I can't remember the details but Lawrence Lessig's excellent book Free Culture includes a chapter about a kid who augmented a search engine for his university network and was taken to court for vast sums of money by one of the music publishing companies I think. All he did was fix the bugs in an existing search interface, and was not responsible for any of the content being shared. But they came down on him like a ton of bricks. If I was being cynical (and I always am) I'd say they wouldn't dare with Google though... The book is freely available (Creative Commons licence) so you can get and read (or even listen to) it from many places online.