So, postmodernism is something that's notoriously difficult to pin down. I typically try and use the idea of pastiche for my students, to give them a general sense of the post-modern self-referential viewpoint. As an example, I've been using Scream, especially contrasting it with movies like Scary Movie (pastiche vs spoof).
So, I saw Tropic Thunder tonight. Wow. I am actually amazingly impressed by the levels of humor and pastiche that are going on in this film. And it's more than just Robert Downey Jr.'s performance (which is excellent). The whole film has wonderful references, Apoc Now, Private Ryan, Tootsie, River Kwai, a Nick Nolte reference that is not done by Nick Nolte, and a set of "trailers" which completely tell you everything you need to know about the main characters without any exposition whatsoever.
Oh yes, and Tom Cruise puts on one of his best performances. Ever.