Intrade is just one such place to bet. See others listed here: http://www.oddschecker.com/specials/politics-and-election/us...
SkyBet and Ladbrokers for example both have payouts closer to 538's forecast.
What is peculiar is that Intrade is the outlier. In a liquid market this shouldn't happen because it allows for risk-free payouts. So something seems a bit odd about Intrade.
It's possible the campaigns have caught on to Intrade's impact on the media and will now step in when prices fall to much. </conspiracy>