The Arisocrats is a movie about one joke, breaking the record for "Least amount of jokes in a movie", previously held by Team America.
Wikipedia explains the joke well, but here's the basics: it's a non-joke. An in-joke among comedians. The start is always the same, the punchline is always the same, so the joke is all about the creativity put into the depraved, obscene stuff in the middle (I'm no expert, but I'd say that the South Park guys currently hold the record for obscenity).
And so to the movie - a documentary featuring 100 comedians explaining their own personal version of the joke, as well as the history of the joke, and the relevance in a society at odds with the "limits of free speech". The movie was co-produced by Penn Jillette (of Penn and Teller fame) and features, among other things the amazingly obscene George Carlin, the amazingly funny Eddie Izzard and uh... Carrot Top.
In other documentary news: I know my birthday is months away, but there's an Errol Morris DVD Collection coming out that would look fabulous in my house, don't you think?