Good question. I've been looking for a site like that too. Would love to watch some ski/snowboard/mountain bike/other action sports movies on demand. Hulu has a couple of action sports shows/movies, but nothing really to speak of. Occasionally, you get a single release like Xavier De La Rue's "This is My Winter" (awesome and free), but other than that, I haven't found any good source for ski movies on the Web. The Ski Channel has shows and movies on my cable on-demand, but I don't think they have anything on the Web.
I'll have to check out iTunes. How much are the titles (don't have iTunes installed on this computer or I'd just go look myself)?