Actually harleyb and I have been hashing this out all day.. two rating systems, one for player ratings and one for show ratings. Then if players have a dispute or a problem they can post a negative with an explanation as well as have a dispute resolution system in there - but it would show the transaction process just like what we have come to expect on eBay or the vBulletin rating systems.
Hosts would record their attendances after the game.
If you want to get really strict about it you could set your game up so that only players with ratings above a certain level need apply. I can see people posting their player ratings in their sigs the way they do their trader/buy and sell ratings now.