I've put some thought into predictions market, and getting around money handling and laws are only the first stages of problems you encounter. I think the big one is who makes the call? You'd have to have the reporting agency explicitly stated earlier and basically assert that their judgement is uncontestable. For elected offices that's not so hard, few people will say that, say, the president was incorrectly elected and they question the call made by everybody reporting on it. But just 2 months ago, large organizations were questioning the validity of the unemployment numbers produced by the department of labor.
I guess the hard part is finding a reality all parties can agree too. The fact that I just wrote that makes me sad.