The Diablo III blog has been updated with the news that the ability for players to appear "offline" during game play will be added to the game with the v1.05 patch. The same option will also be added separately to World of Warcraft and Starcraft as well over the coming months.
Diablo III players will be able to select "Appear Offline" as one of their Battle.net social-status options (along with Available, Busy, and Away) for those times when they want to wander Azeroth by their lonesome, dominate the galaxy in radio silence, or slay demons in solitude. When you choose this option as your status, 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 a game.
Read more on the Diablo 3 blog.