Good idea to pick a film early! :)
True, but might not keep the idea. Might change my mind XD
Truth be told, I found that in the grand scheme of things it's fairly irrelevant what you actually end up picking! The most important thing is that you need explain WHY the things in the film are postmodern, not detail every incident of postmodernism that occurs in the film :).
For example (and this doesn't necessarily relate to Snakes on a Plane) it's easy to point out that breaking the fourth wall is a recognisable trait of postmodernism. But the really meat in the argument would be, why is it postmodern to break the fourth wall?