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Diablo 3 patch 1.0.5 will contain an option to allow players to appear offline. This will keep Chatty Cathy types from bothering you while in the middle of being swarmed, or if you just want to be incognito, it’s for that as well. The option will also be available in World of Warcraft and StarCraft 2 sometime after the release of each game’s upcoming expansion.
~ Source