He won't win - he's neck and neck in the pols with McCain, which means McCain wins. Why? Because opinion polls are snapshots of people's voting intentions...but if 45% say they will vote for him come November, on the day itself a certain amount of them (I would guess 5%-8% of the electorate) will change their mind at the last minute and find they aren't "brave"/liberal/open-minded enough to vote for a black man, so they'll switch to McCain as the lesser of two evils.
For Barack to win, he needs to have a 8-10% lead in the opinion polls, which would actually equate to winning by a smallish margin on the night.
I also think he's messed up royally in picking an old man who garnered just 9,000 votes in the Democratic primaries this year, as his VP candidate - how many extra votes will Biden bring to the cause? None. Barack should have gotten over his (possibly egotistical) worries that having Hillary as his Veep candidate would have meant he was playing second string to the Clintons in the publicity stakes - she would have brought enough votes to swing it for him. As it is, I wish her luck with her 2012 campaign....