Traditional radar works by sending out radio signals, listening for those signals to bounce off an object (such as a plane) and using the time taken for the round trip to calculate the object's distance. A new type of detection system, called passive radar, does not transmit signals, it only listens for them. By utilizing television, radio, and cell phone transmitters, along with high-speed computers to sort through the clutter, a passive radar system can detect the way moving objects change the surrounding signals.
Some experts believe these systems could be the death knell for stealth aircraft, which can evade conventional radar but show up as "shadows" on passive radar systems. Others worry this technology could be used for tracking people. According to Roke Manor Research(1), developers of CELLDAR™ (Cellphone Radar System), passive radar technology "can detect vehicles and even human beings at militarily useful ranges." Read more at New Scientist(2).

1. http://www.roke.co.uk/sensors/stealth/celldar.asp
2. http://www.newscientist.com/news/news.jsp?id=ns99994299