Blizzard's new "appear offline" functionality will likely put a smile on your face.
"All of your Real ID friends, BattleTag friends, and character-level friends will see you as offline in their friends list whenever you're logged into the game," Blizzard says on its Battle.net website. This is assuming you don't set your social status option to available, busy, or away, of course. The new functionality is slated for World of Warcraft as well as StarCraft II and Diablo III. WoW's version of the feature should surface sometime after Mists of Pandaria.