According to the DMG, page 303:
For each 200 pounds of an object’s weight, the object deals 1d6 points of damage, provided it falls at least 10 feet. Distance also comes into play, adding an additional 1d6 points of damage for every 10-foot increment it falls beyond the first (to a maximum of 20d6 points of damage).
I read this to mean that the damage added by the distance of the fall caps at 20d6, but the damage based on the weight of the object doesn't have a cap.
A friend of mine says that remembers a lot of discussion on the topic years ago, and remembers a ruling that the damage that a falling object deals has a hard cap of 20d6, regardless of additional weight.
Unfortunately, he can't find where the stuff he remembers was said, and so all we have to go on is the text in the book. Does anyone know what the proper right answer is? Preferably with a source?