Over at the Princeton Election Consortium I have been concerned with factors that may systematically bias polls. Accuracy is an essential component of using national polls to gauge where national opinion is - or, in the case of my Meta-Analysis of State Polls, the Electoral College.
A hot topic among polling nerds is the "Bradley effect," which occurs when a non-white (usually black) candidate falls short of opinion polls on Election Day when he/she runs against a white candidate. For this reason it has been suggested that support for Obama might be overstated - a hidden penalty. Now comes a large-scale empirical study (in preprint form) by Harvard political scientist Dan Hopkins. He finds that since the mid-1990s, the Bradley effect has disappeared. A preprint of his paper is a must-read.