The margin of error of a poll is the margin of error at the 95% confidence level (two sigma). The standard "margin of error" when you are talking about polls.

http://en.wikipedia.org/wiki/Margin_of_error#Different_confi...

In reality, you should apply a finite population correction, which would make the "margin of error" a bit smaller if you get a substantial percentage of the population voting:

http://en.wikipedia.org/wiki/Finite_population_correction#Co...