Apple has issued a firmware update for its latest Retina MacBook Pro systems that adds support for the new PowerNap feature in OS X Lion.
PowerNap allows the system to save energy by maintaining Sleep mode but still allow the system network access for updates and synchronization, as well as allowing backups to happen in the background. This feature is only available on the 2011-2012 MacBook Air systems and in the MacBook Pro with Retina Display; however, these systems did require a firmware update to use the new feature.
Apple issued the required firmware updates for the MacBook Air systems when Mountain Lion was released, but was still working on the update for the MacBook Pro system so owners of these systems have not been able to use PowerNap feature. Today Apple released the firmware for the Retina MacBook Pro, so after applying it users should see a PowerNap enabling option in the system's Energy Saver preferences.
When applying the update, be sure to follow any onscreen instructions, as interruptions in a firmware update can result in a broken and unrecoverable system. Be sure to back up your system fully before applying the update, and have your system attached to a reliable power source. The update should be available through Apple's Software Update service (in the Apple menu), but can also be downloaded directly from the update's Web page.