Preview at WWDC likely in June, followed by September launch.
Apple Launches OS X Mountain Lion via Mac App Store for $19.99
OS X Mountain Lion brings a number of new features and enhancements to the Mac, including improved iCloud integration with support for Notes and Reminders, iMessage support via the new Messages application, Notification Center, Power Nap updating during sleep, Dictation, enhanced sharing tools, Game Center, improved security with Gatekeeper, and more.
“People are going to love the new features in Mountain Lion and how easy it is to download and install from the Mac App Store,” said Philip Schiller, Apple’s senior vice president of Worldwide Marketing. “With iCloud integration, Mountain Lion is even easier to set up, and your important information stays up to date across all your devices so you can keep editing documents, taking notes, creating reminders, and continue conversations whether you started on a Mac, iPhone or iPad.”For users looking to deploy server capabilities, Apple will also be releasing OS X Mountain Lion Server as a $19.99 add-on via the Mac App Store.
Update: Many users are reporting difficulties purchasing and downloading OS X Mountain Lion, undoubtedly due to heavy load on Apple's servers.
Update 2: OS X Server is now available.