Apple introduced Power Nap, a new feature of OS X during today's WWDC 2012 keynote. Available on
the MacBook Air (2nd gen) and MacBook Pro with Retina Display "Mac laptops with flash storage," this feature runs backups and "keeps data up to date" while the computer sleeps.
Apple notes that Power Nap is silent and power efficient. It's supposed to get mail, update calendar entries and more -- including backing up to Time Capsule, if you've got one.