Researchers have submitted a paper for publication (read preprint) detailing how to use WiFi to see through walls. The system works by measuring interference between nodes of wireless devices. When someone walks through the field, the change in resistance is detected and plotted by a central computer. Ars technica reports:
The basic idea behind the work is the assumption that transmissions between any two IEEE 802.15.4 devices travel within an ellipse centered on a straight line between those points. It's not possible to know which path a given transmission took, as different paths will be subject to different interference conditions. But it is possible to figure out when something along the paths changes position, because the signal strength of a subset of the paths will change as a result of the altered interference.
To get any significant resolution requires a mesh of devices and some very clever maths. However, as WiFi devices become ever cheaper and more commonplace in homes and offices, the potential of using them as a spy system grows. For example, I could see this being used as an integrated motion-detector system in home or office environments.
Just remember: it can't see you if you don't move...
To get resolution, you'd need an antenna array that works like holography but at 2.4GHZ instead of optical frequencies. NORAD has radars like this.