Firmware update enables 'Power Nap' feature for Mid-2011 MacBook Airs
Apple rolled out the new firmware via Software Update in conjunction with the release of OS X Mountain Lion after the company previously noted Power Nap may not be able to work on certain Macs without such an update.
First outlined in June during Apple's WWDC, Power Nap enables Macs to stay up to date and allows for the automatic backup of data while the machine is sleeping. The automatic refreshing of data, including Time Machine and iCloud syncing, is reportedly silent and power efficient though compatibility is limited to Macs with solid state drives like the MacBook Air.
When OS X Mountain Lion was released earlier in the day, some MacBook Air users noticed that the touted feature was missing. The problem has apparently been remedied in the Software Update which is for some "MacBook Air SMC Firmware Update 1.5 for the MacBook Air."
Source: The Next Web
After the firmware is download through Software Update the Power Nap feature can be found in the Energy Saver pane in System Preferences.