At least three people have brought lawsuits against James Cameron, or his company, or somebody, because they claim the film Avatar closely resembles certain ideas of theirs, according to this article from CNN and this one from the Hollywood Reporter.
Well, sure. As I pointed out in my review of the film, it eerily resembles a novel I was writing back when I was in middle school. Avatar isn't exactly full of farm-fresh ideas.
Of course, that doesn't mean the claims in any of the lawsuits aren't legit; on that I have no actual basis for judgment.