Preview at WWDC likely in June, followed by September launch.
Apple Expands Testing of OS X Mountain Lion to Select Retail Store Staff
In an e-mail to Apple Store Genius Bar members and Creatives, Apple has provided access to its OS X Mountain Lion AppleSeed testing program…Not only will the program provide additional testers for Apple as it expands beyond registered Mac developers, but it will also give retail store staff a head start on gaining familiarity with the forthcoming operating system before formal training begins.
This testing is to be done on personal Macs belonging to employees, and is not standardized in-store OS X Mountain Lion training.
Apple is almost certainly wrapping up work on OS X Mountain Lion, if it hasn't completed it already, with the company issuing a "near-final" build to developers at its Worldwide Developers Conference earlier this month. With OS X Mountain Lion being a Mac App Store exclusive, Apple can continue to work on the operating system until relatively close to the launch date given that it does not need to build in time for pressing millions of DVDs, but the company will want some time to ensure that the golden master build is behaving properly before it is released to the public.