What happens if the statement can not be decided?
If the moderators eventually decide that statement turned out to be undecidable (e.g. sports event is canceled), they will call it a draw and everyone will get their bets back without any commissions.
The way most statements are phrased, I don't think any of them would be "undecidable". For example:
The Blue Team will win the 2012 Grand Sports Trophy.
If every single team gets killed by a meteor strike (or the event gets cancelled for some other reason), the Blue Team did not, in fact, win the 2012 Grand Trophy. As such, the people who placed a bet on "disagree" deserve the money.
How will you handle situations like that?