Wow! A lot of people "answer" or simply speculate when they have no idea, don't they? I can tell you what my grandmother told me years ago. A single penny left in the room is a compliment, and means that you intend to return. It's pretty old fashioned, but I still do it.
If I were to speculate myself, the "lousy tip for lousy service" theory holds no water, since it's not always customary to leave a tip for maid service anyway.
I do leave a tip sometimes, for exceptional service, but that's in addition to the single penny..