The Supreme Court ruled unanimously Monday the authorities need a probable-cause warrant from a judge to affix a GPS device to a vehicle and monitor its every move.

I think this result is correct and obvious - the act of physically attaching something to a car trespasses the integrity of a private thing (the car) and so is a search under the meaning of the 4th amendment if done in order to obtain information. Alito wrote a concurrence joined by liberal justices in which they offered that the matter should instead have been decided on the basis of the reasonable expectation of privacy that people have in their movements. I'm comfortable with that too, but the conservative case is simpler.

Sotomayor delivers an important solo concurrence in which she attacks the third party doctrine. Judicially, this is unnecessary to resolve this case (and she acknowledges as much), but politically its really important to have a Supreme Court justice making these arguments and it may have an important, positive impact on the future of privacy in the Internet age. She earns a lot of points in my book here:

More fundamentally, it may be necessary to reconsider the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties. E.g., Smith, 442 U. S., at 742; United States v. Miller, 425 U. S. 435, 443 (1976). This approach is ill suited to the digital age, in which people reveal a great deal of information about themselves to third parties in the course of carrying out mundane tasks. People disclose the phone numbers that they dial or text to their cellu- lar providers; the URLs that they visit and the e-mail addresses with which they correspond to their Internet service providers; and the books, groceries, and medi- cations they purchase to online retailers. Perhaps, as JUSTICE ALITO notes, some people may find the “tradeoff” of privacy for convenience “worthwhile,” or come to accept this “diminution of privacy” as “inevitable,” post, at 10, and perhaps not. I for one doubt that people would accept without complaint the warrantless disclosure to the Government of a list of every Web site they had visited in the last week, or month, or year. But whatever the societal expectations, they can attain constitutionally protected status only if our Fourth Amendment jurisprudence ceases to treat secrecy as a prerequisite for privacy. I would not assume that all information voluntarily disclosed to some member of the public for a limited purpose is, for that reason alone, disentitled to Fourth Amendment protection. See Smith, 442 U. S., at 749 (Marshall, J., dissenting) (“Privacy is not a discrete commodity, possessed absolutely or not at all. Those who disclose certain facts to a bank or phone company for a limited business purpose need not assume that this information will be released to other persons for other purposes”); see also Katz, 389 U. S., at 351–352 (“[W]hat [a person] seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected”).

Thats not how our law works today, but I think it would work a hell of a lot better if it did.

Warrants Needed for GPS Monitoring, Supreme Court Rules | Threat Level | Wired.com