Put up or shut up...

Why didn't I think of this?  I give you the first gambling web site for multiplayer games:

http://www.worldgaming.com/

I read about it in an article in EGM.  Basically it's a site where they take your money - you play a match and bet you can win - then they get the match results from the servers and pay the people who won.

They are very strict about what games can be played.  They won't go by what the players say, it must be a game where they can access the game data and determine that it was a fair fight.  They also have a feedback system so you (hopefully) won't play against any whiners or quitters.