In the case of Obama, I think a lot of people being polled are saying they support Obama simply because they're afraid if they don't the poll-taker will think they're a racist.
Those people won't have to worry about anyone's opinion of their vote in the privacy of the ballot box.
Otherwise knows as “The Bradley Effect”...
Putting aside the so-called “Bradley effect”, I really do think there are a lot of the pissed off Hillary supporters out there that will not vote for BO. I saw one interviewed (she started the website: clintons4mccain.com) that was very articulate, passionate and reasoned in why she would never vote for Obama — — he is an empty suit with no experience. Further, a lot of Obama supporters are juvenile, snide and arrogant and have really turned off a lot of these Clinton supporters. I do not think we are done eating popcorn yet...
Dial-up vs. high-speed poll.