There's a World of Warcraft in the New Year forecast on Battle.net discussing future plans for the MMORPG. Included is word that they are looking into ways to allow players to essentially trade gold for playing time through in-game economics. Here's word:

We’re exploring the possibility of giving players a way to buy tradable game-time tokens for the purpose of exchanging them in-game with other players for gold. Our current thought on this is that it would give players a way to use their surplus gold to cover some of their subscription cost, while giving players who might have less play time an option for acquiring gold from other players through a legit and secure system. A few other online games offer a similar option, and players have suggested that they’d be interested in seeing something along those lines in WoW. We agree it could be a good fit for the game, and we look forward to any feedback you have as we continue to look into this feature.