Preview at WWDC likely in June, followed by September launch.
OS X Mountain Lion Bringing Automatic App Downloads to Mac
Like on the iPhone and iPad, when you buy and install an app on one of your Macs, all of your other Macs logged into the same App Store account will automatically install the app too.Apple has been issuing regular updates of OS X Mountain Lion to its Mac developer community, and the company is expected to offer many more details on the next-generation operating system at next month's Worldwide Developers Conference in San Francisco. OS X Mountain Lion is currently scheduled to see a public launch in "late summer" of this year and will bring a number of enhancements including several features drawn from iOS such as Notification Center, Game Center, Messages, and Reminders.
Unfortunately, the feature does not seem to be working completely. While the App Store will still offer to enable automatic downloads (as seen above), it does not actually install anything when you purchase apps from another computer.