The DC-9-15MC had folding seats so they say @ http://www.freightersonline.com/plane.aspx?a=19
A great Idea. Cleaning,vacuming,and evacuations would also benefit from this type of seat design. Airbus are looking at this I believe.
Has any other civil airliners ever had folding seats fitted for pax? It is common for crew seats but not for all the cabin.