"There is no Fourth Amendment violation because Skinner did not have a reasonable expectation of privacy in the data given off by his voluntarily procured pay-as-you-go cell phone," wrote Judge John Rogers, in the majority opinion. "If a tool used to transport contraband gives off a signal that can be tracked for location, certainly the police can track the signal."
Is this a game changer?
Next thing ya know, they won't be checking GPS, they'll be listening in. I'd be surprised if they haven't already started.