Maintain your World of Warcraft, Diablo II, and Starcraft II from your desktop.
By updating your profile beta-test settings on Battle.net, you will be automatically added to a beta-test pool for the new desktop client Blizzard is rolling out. Once in the beta, you will be able to talk to your friends in WoW, Starcraft II, and Diablo II from your windows or OSX machine. It will replace all launchers for Blizzard games but will also automatically update each of those games while you are logged into the client itself. Blizzard indicates that they will be rolling this app out for all their games once it goes live. It is currently unknown when the client will come out of beta but there is not an NDA for this beta-test so expect to hear more news in the future.
Blizzard notes that there can only be one active login for the battle.net client but it can launch multiple World of Warcraft accounts on the same or different systems. The instructions for this are located in this FAQ. The games still on classic Battle.net do not seem to be supported so this means that Diablo, Warcraft III, and Starcraft cannot be maintained through this client and must instead be maintained on their own.
Source: Laucher Update Testing FAQ