Knighthawk News has an interesting article about art direction, with a bit history. Actually, the site has a lot of great columns on the history of the Academy Awards worth checking out:
It used to be quite common for films to get an Art Direction nomination and nothing else, but that has been less common since they did away with the separation of Color and Black-and-White in 1967. Since then, it is just as likely that a nominee get 2 nominations (for Art Direction and Costume Design), than to just get Art Direction.
You could get lost over there at Knighthawk News. I especially liked this article about Oscar’s defunct categories, like Best Comedy Direction.