Preview at WWDC likely in June, followed by September launch.
Apple Begins Accepting OS X Mountain Lion Apps for Mac App Store
Following the release of the Golden Master version of OS X Mountain Lion, Apple has begun accepting Mountain Lion apps into the Mac App Store. From an email that Apple sent to registered developers:
Make sure your apps take advantage of the great new features in OS X Mountain Lion when the world's most advanced operating system ships to millions of customers worldwide. Download OS X Mountain Lion GM seed and Xcode 4.4 GM seed, now available on the Mac Dev Center. Build your apps with these latest seeds, then test and submit for review.OS X Mountain Lion is only available to registered developers presently. The final version will be available to customers through the Mac App Store later in July. Apple will be pricing the release at $19.99.
You can also submit iOS apps that take advantage of the new cross-platform Game Center Groups, so they'll be available when OS X Mountain Lion ships.