I can’t imagine how Intrade would actually be accurate, unless all the players were only there to make a profit.
But clearly they’re not. Some people are willing make enormous donations to their candidate. Nothing stops them from spending whatever amount of money they want to spend to make the Intrade numbers look like whatever they wish. The whole notion of Intrade breaks down unless it is being used to predict events that the traders are guaranteed to have no stake in.
The CEO of Intrade posted a commentary on that sort of manipulation and the short story is that the market absorbs those attempts. Basically you just end up putting money into the pockets of traders.
Alleged Manipulation on Intrade
http://www.freerepublic.com/focus/f-news/2107813/posts