The WSJ had a very interesting article last week Thursday about the Nielsen ratings: turns out these are done on paper, error-prone, and hinted that as a result the ratings for network watching are probably very inflated (the impression I got from the article was that the type of people who are willing to do the nightly Nielsen paperwork are people who have a lot of time on their hand, and therefore not 'average' Americans, are not too aware of what's going on, and many probably don't even have cable or satellite). The networks probably have much fewer viewers than they are letting on right now.
That Nielsen paperwork is a pain in the you-know-what. I only had to do it for a week, but it was awful. I'm surprised anyone bothers to fill it out at all.