Pre-orders and previews begin April 10, launches April 24.
Software Update to Move Inside Mac App Store in OS X Mountain Lion
Pocket-lint has more on Apple's decision:
Currently Apple's Software Update system in Mac OS X 10.6 Snow Leopard, and Mac OS X 10.7 Lion force users to use a dedicated update procedure that constantly checks to see if updates are available. Apple now feels, it seems, that this way of doing things might be slightly confusing to users coming from an iPhone or iPad who are used to seeing updates for apps appear in the App Store rather than buried elsewhere.The report notes that the move appears to require that users sign up for an Apple ID if they have not done so already in order to access the updates, and also encourages them to keep the Mac App Store within easy access in their Docks so that they will notice badges informing them of available updates.
Explaining to Pocket-lint in a behind-closed-doors briefing for the new developer preview of Mac OS X Mountain Lion, Apple has told us that the new method will offer updates to the OS and Apple applications in a similar way to how it does on iOS via the App Store, but in this case via the Mac App Store.