no, Nielsen ratings are purely who watches it on TV. They're effectively a tool for advertisers. The more people watch TV, the more people are exposed to adverts, the more the advertisers pay. iTunes doesn't count cos it doesn't include adverts (I think). It does however provide funding to the studio.
I'll admit to downloading the last 24 episodes. I live in the UK, Don't have a choice. But having downloaded all of Season 2 I then went out and bought the entire season on DVD. I will do exactly the same for season 3 once the DVD comes out.