It is interesting to note that according to this poll the Marriage Protection Amendment leads by a wider margin among people who have yet to vote:

"One week until votes are counted, a constitutional amendment that would define marriage as between one man and one woman leads by a 5:3 margin, according to this latest SurveyUSA poll conducted exclusively for WRAL-TV Raleigh. The measure passes today 57% to 37%, with majority support among men and women, black and white, rich and poor, and in all parts of the state. Opposition is most strong among Democrats and independents, but rises to a majority only among the relatively small number of voters who identify themselves as liberals.

Among the 10% of voters who tell SurveyUSA they have already cast their ballot, the measure leads narrowly, by 4 points. Among those identified as being likely to vote, the measure leads by 22 points. Compared to an identical SurveyUSA poll released 6 weeks ago, little has changed."

This poll took place from April 26-30th.