Many things can be done, especially locally.
Since the accounts feature is basically a local database, you could just re-implement a local database and use bitcoind (or any other service supporting bitcoin) only for receiving and sending bitcoins.
This is what many sites have done, I guess: they at start use accounts because they are handy, and when transaction volume rises too high they reimplement that feature (AFAIK btct.co is just doing or has just done something like that, but I might be mistaken).

Please notice that using bitcoins as an in-game currency would also require modifying the game itself, i.e. it cannot "just replace" the current gold, since the current gold is just a traditional centralised currency fully controlled by them: they can create and destroy it at will. They quite obviously couldn't do it with bitcoins.
So they would either add bitcoin as a parallel currency, or completely redesign the whole game economy.
Hint: redesigning the whole game economy will likely never happen anywhere, at least not in any large project: the best you can expect in existing games is for the first option (i.e. bitcoins added as a parallel currency), more likely bitcoins just used "externally" to purchase game items (and item auctions if you're lucky) or exchanges allowing you to trade game gold with bitcoins (I'm writing one).
I wouldn't hold my breath, though: what you can reasonably hope for is for new games to support bitcoins.