I heard about this movie, created by Penn Jillette (of Penn and Teller), and this "joke" about a year or so ago when this South Park clip was sent to me. As I understand it, the joke is really a gag played by stand-up vets on rookie comics or sometimes on nightclub audiences.
The idea behind the gag is that a hack comic without any real material will often just get up on stage and curse a lot, as if anything dirty is inherently funny. In the practical joke, the test is to see how much the rookie comic (or audience) will laugh at a joke with no punchline.
There are a million variations on how the joke is told, and the upcoming movie shows star comedians with their versions. In all the versions this is how the joke goes: A family is pitching their vaudeville show idea to a venue, and the act they're pitching is a series of outrageously vile acts on one another. After the act is explained, the organizer asks what the family calls their routine. They respond, "The Aristocrats."
How in the world a full length movie can be made out of this I have no idea but apparently it's all supposed to be a statement on free speech or something but so far, if you believe this article, the reviews have been great and audiences have found it really entertaining.
To view the trailer, see the list of comics who perform the joke in the movie, as well as for a better explanation of what this joke is and what this movie is like, visit the official site: http://www.thearistocrats.com/