Apple recently rolled out a firmware update for the 15-inch MacBook Pro with Retina display enabling OS X Mountain Lion’s new Power Nap feature, just like they did previously for the MacBook Airs.

Limited to newer MacBook Air models and the MacBook Pro with Retina display, Power Nap keeps Macs up to date and allows for automatic backup of data while the machine is sleeping. The automatic data-refreshing service, which also handles Time Machine and iCloud syncing, is both power-efficient and silent. Apple’s website describes the feature as seen below:

Power Nap. Mac stays up to date, even while it sleeps.

When your Mac goes to sleep, it still gets things done with Power Nap.3 It periodically updates Mail, Contacts, Calendar, Reminders, Notes, Photo Stream, Find My Mac, and Documents in the Cloud. When your Mac is connected to a power source, it downloads software updates and makes backups with Time Machine. While all that updating is going on, the system sounds are silent and no lights or fans come on, so nothing disturbs you. And when your Mac wakes up, it’s good to go.
From Apple’s release notes for the update:

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 can be found in the Mac App Store while the settings for the Power Nap feature can be found in the Energy Saver pane of System Preferences.

If you aren’t sure if your Mac is eligible for the update, check (and download if appropriate) from the information below:

Macs that use Power Nap

The MacBook Air (Mid 2011 and 2012) and MacBook Pro (Retina, Mid 2012) running Mountain Lion can use Power Nap.
An SMC update is required to gain all the functionality of Power Nap. Here is a list of the supported Macs and the SMC update that is required to support PowerNap:

Model Required SMC update

MacBook Air (Mid 2012) [Download]

MacBook Air (Mid 2011) [Download]

MacBook Pro (Retina, Mid 2012) [Download]
Source: Apple via 9to5Mac