I think an international incident or terrorist attack might change the equation somewhat. At this point in '04, we'd had like 50 terror alerts, although I doubt the alerts alone would work. I fully expect a new video of bin Laden endorsing Obama in the last week before the election.
On a separate note, I think Obama has reached his ceiling. It's hard to do much better than 51-52% with such a highly polarized electorate. I still expect the race to be decided by 4-5 points, which still results in an electoral college landslide.