[AretNCarlsen]

I haven't used any of them, but you could try arbitrary betting sites. Someone has to be willing to take the bet, which would be an entertaining way of putting one's money where one's forum-mouth is. Betable.com seems like the best fit for non-sports actual-money bets.

Longbets.org would let you make an idealistic point. (They don't let you keep your winnings; they have to go to a charity.)

Smarkets.com plays with real money, but seems targeted towards sports and current events.

[adw]

[Disclaimer: I'm an officemate of Smarkets.]

The reason for this is liquidity. You've got to have enough people interested in an event to get effective price discovery, and if you're making money on the commission then it's entirely about maximising traded contract volume.