On Crooked Timber, Brian is teaching a course in time travel and has some fascinating thoughts on time travel movies:
In Back to the Future in 1985 the first time around George works for Biff, and the second time around, after Marty has changed the past, Biff works for George. So this is a violation of the one-dimensionality principle. I had always assumed that the movie could be made sense of on a ‘branching time’ model. Indeed in the second movie that’s exactly the kind of model they say they are using.
He talks extensively about the Back to the Future (and don't miss the excellent comments as well). Where was this course when I was in university?