This is interesting: U.S. Forces may be using "smart dust" in Iraq and Afghanistan according to Strategy Page:
"Smart Dust" is basically very miniaturized electronic devices. This is similar to stuff like RFID, smart cards, EZ Pass and those rice grain size tracking devices you can have injected into your pets. But Smart Dust takes this all to a new level by being small enough to be disguised as dirt, the kind you can pick up in your shoes or clothing. Each bit of Smart Dust can be given a unique serial number that, when hit with an "interrogation signal" from troops on the ground, or aircraft overhead, is broadcast back. Some forms of Smart Dust are believed to be in use in Iraq and Afghanistan. It's also believed that Smart Dust played a role in the recent death of al Qaeda leader Abu Musab al Zarqawi.
In this case, if someone were able to sprinkle some Smart Dust on Zarqawi's clothing, it would have been a simple matter to track him with great precision. Iraqis have already heard of this stuff, but regard it more as "magic dust." Iraqis have a tendency to exaggerate American capabilities, especially when it comes to technology. But U.S. troops have learned to use this exaggerated reputation to their advantage, threatening Iraqis with magical capabilities that don't exist. That often works, just like smart dust.
It doesn't strike me as likely that this stuff was sprinkled on Zarqawi. If we were close enough to him to do that, if we knew where he was when the smart dust was sprinkled, it seems we would have taken him out then. But maybe not.