Yeah, I guess the Serenity example might not be a good one for what I meant, there are other loopholes, but I won't go into them because like I said it is a grey area of copyright law and far too often people misinterpret for their own convenience.

Serenity and BG are from the same effects house.  This was the same for ET appearing in Star Wars (and Steve and George are old friends).  Trivia, did you know the modellers for Close Encounters (having come fresh off the original SW movie) sculpted R2 and C3P0 into the UFO?  Another inside gag/tribute.  They are all over the place, but are usually done by people who worked on both projects.  A real sign of love for a project, if you ask me.

(Goonies pirate ship had R2 on deck another nice inside tribute.  I saw the boat miniature in person and R2 was very hard to find, though, as he is painted to look like all the rigging and such.)