Watch the Game Theory video about the legality of fan games. I believe it's the same laws. Could be wrong though.
I just remember one part where he was talking about this fan game that introduces new mechanics than the original game, and that if the creator of the original game put the mechanics into a new game, a problem may pursue.
More similar to this though, the creator of the original game accidentally used fan art for a poster for one of his games, and then quickly changed it and apologized when he realized.
Actually, that reminds me. There were international posters for Solo where they accidentally used fan art. This is nothing new. I don't remember what came of it though.