Preview at WWDC likely in June, followed by September launch.
Apple Enables 'Power Nap' for Retina MacBook Pro With SMC Update
Apple has issued a system management controller (SMC) update for the MacBook Pro with Retina display, enabling Power Nap support in OS X Mountain Lion.
This update fixes several sleep/wake issues to improve the stability of MacBook Pro with Retina display (Mid 2012) computers and is recommended for all users running OS X v10.7.4. It also enables Power Nap support for users running OS X v10.8 or later.The update is available via Apple's software download webpage, Software Update for those under OS X Lion and via the Mac App store for those with OS X Mountain Lion installed.