Using Wi-Fi to see through walls? Could it be real?

It's hard to pass through a given neighborhood without passing along dozens (if not hundreds) of Wi-Fi networks in virtually every home, apartment, business etc. 

With that many networks, what are the new risks? Is there an actual way to see through walls using Wi-FI? Could that be used for good and bad?

Check out this fascinating interview featuring Peter Marks and Ariel Bogle as they take a look at using Wi-Fi to track movement and beyond.