@markm: It sounds like a really nice idea. If it's something you're planning on creating yourself and need any help or advice on incorporating GameCoin I'll be happy to help with it in any way I can.
It is up to the players what player-to-players currencies they choose to use, not something that needs to be built into a game-engine.
That also creates niches within the game(s) for trusted market-operators and traders and so on, since the game engine itself provides no guarantees you will get what you pay for or get sent the coins you sell stuff for. Putting code directly into the game to handle player-to-player (p2p) currencies just takes away from the players that whole opportunity to build up their business by building trust. Though of course it also eliminates the opportunity for nafarious long-con businesses to build up trust then cheat people.
GameCoin is just another bitcoin-based currency, it is pretty easy to plug in any such currency once you have one coded. I built IRC bots that gave people accounts in many currencies and let them trade them and so on, and the same shell scripts can be used by bots in various games. But if the players aren't bothering to introduce their favourite coins into the games it isn't really a great idea for the game administration to play favourites and pick one or several to provide built in support for.
Maybe when some clan or guild or nation or whatever gets a particular coin really super-well established they could in the game construct banks or whatever, ultimately finance getting code specially to support their currency put into the game via their national bank or whatever. I am thinking for example on the Freeciv scale, maybe when someone builds a bank in their city and gets an Open Transactions server set up to represent that bank, they could choose its main currency or something.
That Ix character who tried to bring Ixcoin trading into the game wasn't even from this forum, in general people from this forum aren't really into games other of course than "playing the markets", they seem to prefer to just bounce around on the web trading at exchanges and such without having a "character" that they play that muggers could lurk in wait for outside the exchange and stuff like that...
Or maybe the hacks and such that have happened to various websites serve as quite enough emulation of such aspects of games.
-MarkM-
