Blizzard decided that they want to start giving you more online control while you’re playing their games. In the very near future, World of Warcraft, Starcraft II, and Diablo III players will be able to have various different social statuses. You can choose whether you’re Available, Busy, Away, or Appear Offline.
This is great for when you just don’t want to deal with other people. Just try not to become that person who is always offline. This will be applied to all of your Real ID, BattleTag, and character-level friends.
This option is going to happen separately for all of the aforementioned games. It’s looking as though Diablo III will be getting it in the 1.0.5 patch, and for WoW and SCII, it will come in an expansion. Feel free to guesstimate when that might be.