http://www.cnn.com/2010/TECH/gaming.gadgets/10/08/fbi.tracks.student.wired/index.html

"His discovery comes in the wake of a recent ruling by the 9th U.S. Circuit Court of Appeals saying it's legal for law enforcement to secretly place a tracking device on a suspect's car without getting a warrant, even if the car is parked in a private driveway."