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.
Woo, I'm back. Been outta town for about five days, and have a ton of messageboard backlog to go through.... won't that be fun? Well, not that any of you are overjoyed to have read this, but I feel sometimes like making my presence known. Or something....