i'm sure there's some crossover, but generally i feel like they're pretty different.

as you said, body position is not at all similar, the bikes FEEL completely different, etc. i think road riding helps with cadence and spinning on a mtn bike, and mtn biking helps with general comfort on a road bike (and vice-versa), as well as overall strength and fitness. they're pretty different though. for example, i'm a total weenie on mellow, predictable (and SMOOTH) road declines, and yet i'll blast down steep slopes littered with rocks and general instruments of death on my mountain bike.

to me, road riding is infinitely scarier than riding DH or XC. my road bike is so light and the tires are SO skinny, and i feel like i'm constantly fending off traffic, potholes, sticks, etc., etc. i'm not saying one or the other is better, just different. and also pointing out how scared i am on my road bike compared to my cushy, squishy and forgiving mountain bike for the benefit all you more competent road riders who think mountain biking is scary.