So, how do the polling data compare to the contract prices from Intrade on the day before the election? Below is a graph with a data point for each state, with the horizontal axis representing the polling data and the vertical axis representing the Intrade contract price.
polling data & averaging courtesy of Electoral-Vote.com
I flipped a coin and made the polling percentages positive for McCain and negative for Obama. If the poll was 43/51 in favor of Obama, that reads as -8 on the x axis. Likewise a 52/49 poll in favor of McCain would read as +3 on the x axis.
The cost of buying a contract for a McCain victory is on the Y axis. (An Obama-victory contract would cost roughly 100 minus that value, as the market does a pretty good job of keeping the two contracts trading opposite each other.)
The blue shaded region is where both Intrade and the polls favor Obama. Likewise, the red shaded region is where McCain is favored in both the polls and at Intrade. Any data point outside of a shaded region represents a disagreement between Intrade and the latest polling data.
The pink curve shown is the result of a logistic transformation linear regression, with the intercept forced to zero.