Have you ever heard of 'Smart Dust'? Well, the military has had it in development for the last ten years. Basically its a miniaturized RFID like device about the size of a rice grain that can be used for tracking people.
The smart dust can be made to look like dirt, and can be placed on people's clothing. Each device has a unique id number assigned to it, that can be track by troops or aircraft when sent a special 'interrogation signal'.
Smart dust is believed to have been used in Iraq and Afghanistan. It was also believed to have been used in the recent death of the al Qaeda leader Abu Musab al Zarqawi.