Despite the apparent/deceptive closeness in the polls, the odds are running about 60/40 in Obama's favor as the the likelihood he will beat McCain. The trading market represents a consensus opinion of where traders see the election going and the "stock value" of each candidate.
These trading markets (while not always perfect predictors) do represent a "collective wisdom" of experts and amateurs and have made some good predictions in the past, that were missed by other techniques.
BTW, GOP VP odds running about 70/20 for Romney vs Pawlenty
Here's the latest Presidential odds (this is link to InTrade, so it should stay up to date).
Obama:
McCain