Important question about exit polls...

Does anyone know how they get the national results? Is it a completely separate poll or do they just take the state polls, weight them by the number of voters in each state, and use that to come up with national figures? The reason I ask is that there are questions on the national poll that are not found on the individual state polls. Specifically, I wanted to see how states that had "marriage protection" issues on the ballot compared to nearby states that did not have such issues. That question is on the national poll but I don't see it on the state polls. Perhaps they ask only a subset of respondents that question and it is not a high enough number to extrapolate to states, but nationwide it is enough to extrapolate for the whole country?

My guess is there are swing counties that can predict very precisely the outcome in the whole state.

I saw this on CNN on election night. David Grogen (sp) was talking about a county in MO. He said that if a democrat can win this county by 6% he will most certainly win the state. At that moment Kerry's winning margin in the county was exactly 6%. Kerry ended losing MO.

I saw an article somewhere but can't find the link right now. Each state has counties like that that mirror the whole state's results. So, exit polling in these counties only makes sense.