In this post, August talks about watching a recent episode of Lost where one of the characters climbed through an air vent to escape a locked room. August is sick of this plot device:
"I've lived a fairly adventurous life. I've travelled to five continents. But the only time I've seen the inside of an air duct is television and movies, when a character -- generally the hero -- has to be clever enough (and small enough) to climb through a convenientally-accessible air duct ... be it action-adventure, comedy, or horror, the air duct has become the hack screenwriter's go-to passageway."
He's right, of course. Just relying on memory and not checking the web, I can think of several movies where the air vent is a main plot device: the first Mission: Impossible, the first Die Hard, and on television, every other episode of Voyage to the Bottom of the Sea. Though it was pretty funny in Top Secret!