Preview at WWDC likely in June, followed by September launch.
OS X 10.8 Mountain Lion Developer Preview 2 Seeded to Developers
As noted by The Next Web, Apple has also posted an extensive list of known issues alerting developers to various bugs and missing features that have yet to be addressed.
OS X Mountain Lion is Apple's next major operating system version and is scheduled for release in "late summer". The release will offer greater integration with iCloud, as well as a number of new feature including some drawn from iOS.