|Image via Wired|
Justice Antonin Scalia wrote that the court holds "that the government's installation of a GPS device on a target's vehicle, and its use of that device to monitor the vehicle's movements, constitutes a 'search,'" and therefore violated the individual's Fourth Amendment rights.
At least you know now that police can't put tracking devices on your car without a warrant. But this only applies to the US, though. Sorry foreign country readers.