Just yesterday a Continental Airlines flight inbound to Houston from Brazil encounter Clear Air Turbulence. The airplane was a B-767 with 300 passengers.The flight suffered several injuries from people who didn't have their seat belts on at the time. Clear Air Turbulence is an extremely rare event mostly encountered near swift wind changes around the jet stream. It's difficult to detect or predict. The only way to sometimes avoid it is when another aircraft has already encountered it & radio back its position to aircraft behind it . It goes back to the point of obeying the seatbelt warnings you get from the cockpit & having it on even when your sitting down in your seat. I see everyday passengers ignore crew warning. Sometimes it can get frustrating for flight crews. Turbulence is a natural state of the atmosphere & has never caused a plane to crash. However, many people have suffered injuries from turbulence from not obeying crews.