I tried uploading a movie in MPEG 4 format with the extension .mp4, but when I linked to it with [[media:movie_name.mp4]] it just came up as gibberish, which usually would mean that the mime time was wrong. I'm pretty sure that giving it a .mov extension would work fine so long as the user has Quicktime 6, but that's not really as correct as mp4. (And there are probably players out there that can handle MPEG 4 but not quicktime movies in general.)

Nathan