The U.S. Supreme Court ruled today that the Fourth Amendment does indeed preclude law enforcement from attaching GPS tracking devices to vehicles without a warrant. While this is the theme of the rare unanimous decision, the actual language is more nuanced. The government's position was that a GPS tracker was not a search�, but the court disagreed.
In the case at issue, the government claimed to have probable cause, but had not obtained a warrant. Due to this oversight, the Supreme Court opted to withhold a decision on whether or not a warrant is always required for this type of search. From the language of the decision, it appears that the month-long surveillance in this case should have required a warrant in the court's opinion, but shorter periods of surveillance might not. Five of the justices did affirm that physically attaching a GPS tracker to a vehicle was a search, though.