We all have our friends online and we enjoy gaming with them, but every now and then you just want to have some quiet time to get a little gaming in without having to play answer a tell every few seconds. Blizzard will be adding that option to their Battle.net account system in the coming months. The feature will be added in sepeate updates to World of Warcraft, StarCraft II and Diablo III, allowing players to simply appear offline if they so choose. Diablo III players should see the feature go live with the upcoming 1.0.5 update if all goes as planned.
Source: Diablo III website