This already sort of exists in the form of Dragon's tale. However, the gameplay is basically centered around gambling, so it's not a traditional mmorpg.
Spells of Genesis appears to be the first game set to release with heavy Bitcoin integration and usage. If it is successful, it is very possible that other developers may give it a shot with a more complex game, which could ultimately lead to a full mmorpg using Bitcoin.
Yeah and if it will really click. Just like Ragnarok and other mmorpg games that really became famous that will have a really big bitcoin buyers. That will make bitcoin really high because of its popularity and demand.