Composite technology today is leap and bounds from what it was in the 80s. You already trust them, you just don't know it. Everytime you board an airplane, you put your life in the hands of carbon fiber. They are developing new resins every day to reduce the brittleness, increase the impact resistance, and make stronger the carbon fiber composite. Carbon bikes go through more rigorous tests in the bike industry than any other. PS, they are making and using carbon dh bikes. People crash very very hard on dh bikes. Running into trees at 40 mph, flipping over into bushes, etc. The cost of the carbon frame is what is prohibitive and the stresses are greater for mtb than road. It takes a special development to ensure reliability. As light weight doesn't matter for dh, and it is easier to manufacture and develop a metal frame, most go this route.

Still, http://www.trekbikes.com/us/en/bikes.../session_9_9/#, it is becoming more prevalent. For steel to have even close to the same weight as a carbon frame, it would have to be ridiculously thin. At that point, thin carbon, thin steel, and thin Aluminum all crack with impact. Does anyone ever intend to crash? It's not going to come even close to breaking in use.

http://www.youtube.com/watch?v=Y_O9PLorYPA