Before you get too excited about Power Nap, you should know that it requires a "Mac notebook with built-in flash storage." According to Apple, this means the MacBook Air (Late 2010 or newer), the MacBook Pro with Retina display, and that's it. (Even then, it may require a firmware update.)
The SSD requirement makes sense when you consider that Power Nap also keeps the cooling fans turned off during DarkWake. A spinning hard drive executing a full Time Machine backup definitely needs some active cooling. Power Nap requires "built-in" SSD storage because solid-state drives meant to replace traditional spinning hard drives have very different power requirements. So even if your (non-Retina) MacBook Pro came from Apple with an SSD, Power Nap remains unsupported.