The Association of Independent Creative Editors ran a competition in 2005 for people to re-edit movie trailers re-cutting the film's visuals but changing the genre. The winner was "Shining" - Stanley Kubrick's horror movie re-cut as a romantic comedy (there's lots of information on Shining on the tattered coat's blog):

Since then lots more of these have emerged. My personal favourite is "Must Love Jaws" - Jaws re-cut as a love story: (thanks enda.)

Another great one is Caakth?l - Cocktail re-cut in the Bollywood style:

There's a whole website here dedicated to these trailer mash-ups.