Greg Mankiw calculates the probability that McCain will raise taxes, using data from the Intrade prediction markets, which have contracts for who will be elected president and for what tax rates will be in the future. It seems pretty likely.
Of course, the error on these markets is pretty high, and the calculation requires subtraction, which just amplifies these errors. But it's an interesting thought. (And I have to admit I've played around with trying to extract conditional probabilities from prediction markets myself.)