I'd not go for anything with bellows and ground glass for mtn. biking. (Then again, maybe it depends what you mean by mountain biking....)

I'd suggest the fotoman, or if you have lots of money to blow, maybe the alpa. (Isn't that alpa site amusing?! Wish I were actually shopping when I visit it)

Better yet... as few moving parts as possible... a 4x5 pinhole camera! You need a long break from the athletic activity anyway

Tripod is no prob, the bike can be a tripod. Put a stand on the bike, put a tube inside the stem to the seat, pop off the seat, voila.