Clay Porter's films are very professionally done, if a tad dull. The one I've seen, F1rst, cobbles together a narrative around the year before last's DH season, the one where Sam Hill whupped everyone silly. The bloke's narration gives me hives (he sounds like monotonous radio DJ from Reservoir Dogs) and I hate the music he's chosen (overly polished rawk) but it's beautifully shot, especially the floodlit night sequence of the Athertons, and gives more of an insight into what it's like to be a DH racer than stuff like the Earthed series. You can watch it for free in crap quality here:
http://video.google.com/videoplay?docid=-8947017369366051826#
If you can stretch the definition a bit, Joe Kid on a Stingray is a great film about the birth and development of BMX. Tinker Juarez is in it so it's kind of MTB related…