Good question. There seems to be a differing view on what happens during lagering floating around here and I was thinking about posting a thread to see if we could get it sorted out. Some think the yeast are involved. Some seem to think it is a chemical reaction with certain things precipitating out.
Chris White states in his book that yeast won't do much once you get below 40. I've seen some say that the precipitation happens faster the lower the temp.