To the dismay of some users, Power Nap -- Apple's whimsically named, sleep-like power state which quietly polls for updates -- was curiously absent from Mountain Lion's official release yesterday. Today though, the company rolled out an SMC update which adds Power Nap to recent, compatible Macbooks.
Similar to Intel's Smart Connect, Power Nap is a low-power state which provides a minimal amount of power to periodically download new email, notifications, sync calendars, reminders, contacts and more. While on AC power, the power state also allows downloading and installing security updates, backups, indexing and other battery-draining tasks, all while pretending to be asleep. During these tasks, power-napping Macbooks will appear as though they are on stand-by to their owners; however, between those updates, Macbooks will effectively be asleep.
The list of Power Nap-compatible Macbooks is a short one. Mid-2011 and 2012 Macbook Airs qualify while the Retina Macbook Pro quietly makes the cut. One of the requirements for Power Nap is all-flash storage, which possibly accounts Apple's narrow model selection. By comparison, Intel's Smart Connect happens to also be very particular about what hardware it'll work with too, limiting compatibility to Ultrabooks with Intel Wi-Fi cards and SSDs.
Users have reported the SMC update makes their Macbook sound like it is about to lift-off from Cape Canaveral, at least during the installation process. Don't worry though -- this is typical with many firmware updaters and the fan noise should return to normal after rebooting.
A full list of what Power Nap can do can be found on Apple's support website.