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.
Even more proof that MTV sucks. Practically *every* cartoon they've ever made has been totally awesome (a rarity on MTV,) and lasted for a way too short amount of time. Adult Swim needs to pick up 3South and Clone High and drop a few days of anime.