'The Future of Ideas'
In the process of making a film, a director must "clear rights." A film based on a copyrighted novel must get the permission of the copyright holder. A song in the opening credits requires the rights of the artist performing the song. These are ordinary and reasonable limits on the creative process, made necessary by a system of copyright law. Without such a system, we would not have anything close to the creativity that directors such as Guggenheim have produced.

But what about the stuff that appears in the film incidentally? Posters on a wall in a dorm room, a can of Coke held by the "smoking man," an advertisement on a truck driving by in the background? These too are creative works. Does a director need permission to have these in his or her film?
http://www.nytimes.com/2002/01/06/books/chapters/06-1st-lessi.html