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.
I have a sudden urge to start grepping maillogs... However that urge is tempered by the knowledge that this is a spammer I no longer have to worry about; as opposed to the asshole who currently has 40 connections clogging up my mailserver right now.