Here is a pretty good article from Nate Silver breaking down the polling for the 2012 Election. It is important to recognized that 538 is not a polling firm, but an aggregate. It uses all of the available polling information to get to what the election results will look like. This isn't some kind of testament or InYerFace.gif post, but a good look at where things are going with the types of bias in the numbers and a scratch at the surface as to why.

Which Polls Fared Best (and Worst) in the 2012 Presidential Race - NYTimes.com