So my guess was wrong. Clinton won. The question is, was it the various selection biases and undecided voters, the measurement error (polls were on Saturday and Sunday but the election is on Tuesday), or something else?
Unfortunately, there is really no way to tell. However, there are two interesting things to note.
One is that in one local poll, they show the percentages both including and excluding those leaning toward a candidate but are undecided. In this case, Obama received 2% more of the vote if the leaners are counted, indicating there is some bias in the method that most polls use, which is to count the leaners (who are actually still undecided) rather than exclude them (the polls do exclude the truly undecided, which has hovered in the 5-10% range, but implicitly assume they will vote the same way as the decided voters).
The second thing to note is that the polls very accurately predicted Obama's percentage and inaccurately predicted Hillary's. Obama's actual percentage was 36%, and the seven major polls predicted 36%, 35%, 39%,41%,34%,38%, and 39% (an average of 37%). Hillary's percentages (same web page) were 28%, 34%,28%,28%,31%,29%, and 29% (average of 30%) versus her actual of 39% (a difference that is outside of the zone for mere statistical error). Hillary apparently picked up votes from undecideds, people who were leaning for Obama, and from the Edwards/Richardson camps.