Blizzard is planning to update Battle.net with an option like Available,Busy,Away. The option will make you to appear offline to your friends every time you log in at Diablo 3,World of Warcraft or Starcraft 2. The "Appear Offline" option will be added sometime after the update 1.0.5 of Diablo 3, and expansions of World of Warcraft and Starcraft 2.

Source: Battle.net