Blizzard Entertainment Trademarks “The Dark Below”, Diablo III Expansion?

Since we already know that Blizzard is hard at work on at least one Diablo III expansion pack and the game is getting a much-hyped console release soon, I’d be willing to put some money down that this report on Joystiq regarding the studio’s trademark for a game called “The Dark Below” is strong evidence that we have a title for the controversial action RPG’s first add-on. Or I suppose it could be for World of Warcraft, too:

Is this the new Warcraft expansion? Or the Diablo 3 expansion? Or something else entirely. We have zero idea of what this actually is, but hey — that doesn’t mean it’s not something to look at and speculate on. It is interesting that “The Dark Below” exists in WoW via Blizzard’s non-canonical RPG manuals and via a brief mention with Ozumat.