Preview at WWDC likely in June, followed by September launch.
Apple Issues Update to OS X Mountain Lion Developer Preview 3
Apple issued a software update for OS X Mountain Lion Developer Preview 3 this evening. Apple released Developer Preview 3 to developers. The update brings the build number of Mountain Lion up to 12A193i (from 12A178q).
If you already have Mountain Lion Developer Preview 3 installed, choose Software Update from the Apple menu to download the update from the Mac App Store. Download Xcode 4.4 Developer Preview 4 from the Mac Dev Center to continue building apps for Mountain Lion Developer Preview 3 Update.One change noted by 9to5Mac is the addition of a "Do Not Disturb" setting to Mountain Lion's Notifications Center:
Besides the Notification Center, Mountain Lion also borrows a number of other features from iOS, such as Reminders, Messages, and Game Center. Apple first previewed Mountain Lion back in February and promises deliver to customers in late summer 2012.