Wow, I can speak to this directly.
The national sample has been doubled to 10,000 people through expanding the National installed home base, and incorporating the large local markets (NY, Boston, LA, Chicago, D.C., etc.) that have been hooked up with people meters. So, now, the national sample consists of ~7,000 nationwide homes, plus the data from the large local markets). There are also probably closer to 25,000 installed homes in Metered Markets (cities such as Charlotte, Phoenix, Richmond, VA...cities that aren't big enough to support local people meters). The local Metered Markets are how your local Fox, CBS, ABC, NBC, UPN, WB affiliates get their Ratings. The sample is installed to very closely resemble the Universal Estimate (i.e. x% of homes <35 years of age, y% homes 35-54 years of age, z% homes 55+ years of age).