The Battle.net support pages were recently updated to include a section on uninstalling a "Battle.net Desktop app," and lists versions of the client for both Windows PC and Mac. Blizzard's job pages have also added a listing for a software engineer specializing in Mac to "help create our next generation Battle.net desktop client for OS X."
MMO Champion posted a few dozen screenshots previewing the client's launcher, news pages, loading and setting screens and different user avatars and backgrounds, as well as a handful of client strings for the app. The news feed appears to be RSS-style and includes information on all of Blizzard's current titles, including World of Warcraft, Diablo 3 and StarCraft 2. All games appear to have individual launch pages that will post updates, maintenance schedules, patch notices and leaderboards. It also looks like users can customize which games they receive updates for and how they are received.
Another page in Blizzard's support forums suggests there will be a beta test for the desktop app as well.
Polygon has reached out to Blizzard for more information on the Battle.net desktop client and will share more details as we have them.