9-0 unanimous that the police must have a warrant prior to using a GPS vehicle tracker. The reasonings differ among the justices but the vote was unanimous.
The split in reasons did lead to the fundamental question being sort of dodged. The majority opinion decided the case on quite narrow grounds: that the physical installation of the GPS device violated the 4th amendment, because installing it involved tampering with someone's personal property (the car) without a warrant. Four justices would have gone further and argued it would require a warrant to operate such a GPS tracker regardless of how it was installed, but the majority left that question unanswered.