Weighing in on JK Rowling and Warners' lawsuit against a fan-compiled concordance of the Harry Potterverse, Neil Gaiman (whose first two books were unauthorized nonfiction and relied heavily on fair use) describes the creative importance of the freedom of rip each other off in fantasy lit:
Back in November I was tracked down by a Scotsman journalist who had noticed the similarities between my Tim Hunter character and Harry Potter, and wanted a story. And I think I rather disappointed him by explaining that, no, I certainly *didn't* believe that Rowling had ripped off Books of Magic, that I doubted she'd read it and that it wouldn't matter if she had: I wasn't the first writer to create a young magician with potential, nor was Rowling the first to send one to school. It's not the ideas, it's what you do with them that matters.
Genre fiction, as Terry Pratchett has pointed out, is a stew. You take stuff out of the pot, you put stuff back. The stew bubbles on.