I went to the Neilson's website once (I forget what it is now) looking for the same thing.
They said they dont disclose ratings like that because they dont want that information to sway anyone's viewing habits.
For example, if you usually watch SHOW A on Monday nights, but find out that SHOW B's ratings are three times higher than the show you usually watch, you might be influenced to watch show SHOW B to see what all the hubbub is about.
This is not quite as classic as the one couple who argued over whether Peg Bundy or Kelly Bundy was the most attractive. As I recall, the argument became so heated that the husband ended up killing the wife.