Nielsen is reporting that the combined overall household rating in 55 out of the top 56 local markets for last night's presidential debate between Barack Obama and John McCain was 33.2. A ratings point is equivalent to one percent of the homes for the audience measured, so 33.2 would mean the debate was watched in one-third of all television homes.
St. Louis was the top local market in terms of Household rating with over 52% of the homes in St. Louis tuned into the debate. The list is heavily skewed to cities on the east coast (as on the west coast it aired between 6pm-7:37pm) with Denver being the top market west of the Mississippi with a 39.7 share. Not surprisingly my birthplace, Washington, DC was in the top 5 markets in terms of the percentage of homes tuned in. It was #4 with 44.6 % of the homes in that area tuned in.
San Francisco, home of TVbytheNumbers.com, and the sixth largest Nielsen market overall (when you combine it with San Jose, as Nielsen does) was 38th in terms of percentage viewing. But even with the early start, 32% of the homes in the San Francisco Bay area were tuned in with 62% of the homes watching television watching the debate. You can see a full list of household rating/share and number of homes for the 55 markets here (PDF) file.
According to Nielsen this measuring coverage for airings ABC, CBS, NBC, FOX, PBS, CNN, Telemundo, Telefutura and BBCA.