Stirring risotto frequently agitates the rice and causes some of the surface starch to rub off and thicken the sauce. A wood spoon has a rougher surface which also would provide friction during the stirring process to rub off more surface starch.
While all this is true, I'm not sure if you could tell the difference between a risotto made with a wood spoon and one made with a metal spoon or other non-wood implement.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan