The Supreme Court has handed down their decision in the case of United States v. Jones, the case involving attaching GPS tracking devices to vehicles.
The court held that, yes, attaching a GPS tracking device to someone’s vehicle is a search under the Fourth Amendment, and yes, this does require a search warrant. The decision is here.
Even better: it was a 9-0 ruling. (Five of the justices signed on to the majority opinion; the other four wrote a separate concurring opinion.)
Here’s preliminary coverage from Wired‘s “Threat Level” blog. I haven’t had a chance to review the full ruling yet; I may have additional links or commentary later.
Edited to add: Correction. According to the WP, there were two concurring opinions; I missed one in my quick skim of the ruling. Alito, Ginsburg, Breyer, and Kagan signed on to one concurrence; Sotomayor wrote a second concurrence, and also signed on to the majority opinion.
Edited to add 2: Discussion at the Volokh Conspiracy. Part 1. Part 2. Part 3. Part 4. Still wading through this, but part 2 makes a good point; the Court did not actually rule per se that GPS tracking requires a warrant, but that it is a search under the Fourth Amendment. Not all searches require a warrant (see Terry v. Ohio or Chimel v. California) but, in general, the types of searches that the Court has held do not require a warrant involve “exigent circumstances” and are highly limited in scope. I find it hard to see the Court applying an “exigent circumstances” exception to GPS tracking in just about any case.
ETA3: And one of Volokh’s commenters makes a good point: the police apparently had a warrant, but botched the execution of it (“The warrant authorized installation in the District of Columbia and within 10 days, but agents installed the device on the 11th day and in Maryland.”) and then tried to argue that they didn’t need no stinking warrant anyway.