The Times has a piece how people are using GPS devices to track other people by surreptitiously attaching trackers to cars. If the person who places the GPS has an ownership interest in the car, it would probably be legal.

This bit, though is fascinating:

Jimmie Mesis, a private investigator in New Jersey who, with his wife, Rosemarie, publishes PI Magazine and also sells devices through a company called PIgear, recalled a couple whose 17-year-old daughter had a drug problem and would disappear for hours at a time. Worried that she might overdose, they placed a tracker on her car. When they saw that she was visiting the same house repeatedly, they informed the police, who raided the drug den.

Why would the police need a warrant when private parties are already doing all the surveillance!