Diablo 3 will be using two auction houses, one for in-game gold and one for real cash. It will be interesting to see how that plays out. On the one hand, it's a way to cut Blizzard in on the action of (currently) illicit cash trading that goes on anyway, so I suppose that's a smart business move. On the other hand, it legitimises the gold farmer sweatshops. I predict a lot of ordinary players will attempt to buy things with in-game gold, but sell things for real cash, which might unbalance the system a bit, but eventually people will probably settle mostly on one or the other.
I suppose one of the good things about the cash market will be the way it levels the playing fields for gold farmers. At the moment, you have to know where to go to buy gold (or other goods), you have to pay that third party, then meet up in game to make the exchange. If you're a small gold farming business trying to break into that market, you have to spend a lot of time and effort to get noticed and gain trust. If it's part of the in-game market, all you have to do is compete on price, so you could be one guy in his basement on weekends or a warehouse full of orphans operating 24-7 and you both get the same chance to sell your wares legitimately for real cash. Who knows what kind of effect that will have on that business model?
Mokalus of Borg
PS - It might collapse that economy overnight.
PPS - Or it might stabilise it fairly permanently.