I have a MacBook Pro w/Retina running Mountain Lion with the latest firmware update. Power nap does as advertised with my machine. It will do Time Machine backups while the laptop is asleep and it does appear to get email while asleep too (though this is harder for me to establish since the newest emails load almost immediately upon Mail's launch on this machine with my fast wifi connection).
What I'm wondering (and unwilling to test for fear of a corruption), is whether I can close my laptop's lid (or force sleep from the Apple menu) while Time Machine is performing a backup?
According to this support page (http://support.apple.com/kb/ht5394)
When your compatible Mac goes to sleep in Mountain Lion, Power Nap still gets things done silently
- Mail. Receive new messages.
- Contacts. Your Contacts update with any changes you may have made on another device.
- Calendar. Receive new invitations and calendar updates.
- Reminders. Reminders updates with any changes you may have made on another device.
- Notes. Notes updates with any changes you may have made on another device.
- Documents in your iCloud account. iCloud pushes any edits you made to a document to your Mac notebook.
- Photo Stream. Your Photo Stream updates with new photos from your iPhone, iPad, or iPod touch.
- Mac App Store updates. Your Mac notebook can download updates from the Mac App Store.
- Time Machine backup. Your Mac notebook can back up while it sleeps.
- Find My Mac. Locate a lost Mac notebook even when it’s sleeping.
- VPN on demand. Corporate email updates securely.
- Mobile Device Management. Remotely lock and wipe the computer.
But also note, the following (from the same source):
"Power Nap does more when your Mac is plugged in to an AC outlet
When your portable Mac is connected to a power source, it also downloads software updates, makes backups with Time Machine, performs Spotlight indexing, and can continue Mac App Store downloads. It also updates Help Center."
So, it is likely that TimeMachine will only work with Power Nap if it is connected to an AC outlet.