You guys really crack me up, especially eensor and Lexy.

Well, we have not yet experienced this problem, though I have a sudden urge to go examine the walls in the house just be sure.
Do you think they are missing something in their diet that makes them do this? Some children do this (eat plaster off walls) and it is thought to be due to a nutritional deficiency such as iron deficiency anemia or zink deficiency. It is called pica. I wonder if yorkies could have similar nutrional deficiencies? Of course it could just be normal "yorkie" behavior.
On another note, I have a friend whose yorkie was always chewing the wooden legs of her dining chairs, and she bought some of those wooden popsicle sticks that you can make crafts with and put those on the floor underneath the table and her baby chews those up now instead of the chair legs. Everyday she picks up the pieces of the chewed up ones and puts fresh ones down. Sounds strange to me.....but I guess it works.