My thoughts: I once attended a writing class by a man who had written published books and screenplays. He said that Hollywood steals copiously, and without remorse all the time.
Secondly - I've written a series of children's books (as yet unpublished). The first thing they tell you to do before writing a children's book is to read every children's book in exsistence - to see what's out there. I can imagine the Nemo people reading and finding everything they could about fish as research for their screenplay and movie. There have been blatant cases of theft before so it wouldn't surprise me at all. I think it happens all the time. It would be interesting to see who had PIXAR had the original idea and where they got it.