Snakes on a plane is the first movie that is really utilizing viral marketing to it's advantage.
Not only did they put together and ingenious flash application, that creates custom messages that then call your friend, but they had a tag line created by the blogosphere that eventually made it in the movie.
I'm not sure who's in charge of their marketing, but whoever it is, has got it. I've seen some feeble attempts from other movies at this type of marketing, namely Superman Returns creating a MySpace page. This is on a whole other level. The best part about is no one is talking about how good the movie will be.