The main danger is between GA light aircraft and gliders. Being an airline pilot and a glider pilot I would say that the possibility of conflict between an airliner and a glider is minimal. Airliners usually climb rapidly through the lower levels and their departure routings are well known to glider pilots. However the Coventry approach scenario mentioned earlier is more likely where a GA aircraft is climbed to avoid a 737 with the potential conflict of the GA aircraft with a glider.
The main point to come out of this thread is the need for educating power pilots that they could conflict with gliders in cloud. I have to say that when I am gliding it is apparent that power pilots don't keep a good enough look out in VMC, let alone the problems of look out in cloud! When you are close to cumulus clouds at lower altitudes keep a good look out, there will be gliders around!
Also I would not consider flying in CB's in a glider or an airliner! It's those fluffy white cotton wool clouds that glider pilots go for!