Private citizens are using GPS devices is it legal?

But today, anyone with $300 can compete with Jack Bauer. Online, and soon in big-box stores, you can buy a device no bigger than a cigarette pack, attach it to a car without the driver’s knowledge and watch the vehicle’s travels — and stops — at home on your laptop.

Tens of thousands of Americans are already doing just that, with little oversight, for purposes as seemingly benign as tracking an elderly parent with dementia or a risky teenage driver, or as legally and ethically charged as spying on a spouse or an employee — or for outright criminal stalking. So many suspicious spouses are now doing their own spying, a private investigator in New Jersey said, that his infidelity business is declining.
In the absence of legislation in most states, putting a GPS device on a spouse’s car, or hiring an investigator to do so, is widely considered to be legal if the person placing it shares ownership of the car. But some privacy experts question this standard, and there is little to stop a jealous suitor, or an abusive man trying to prevent a battered woman from escaping, from doing the same.
GPS trackers are increasingly being cited in cases of criminal stalking and civil violations of privacy.
   One increasing use of GPS tracking — by as many as 30,000 parents, one seller estimates — is to monitor the driving habits of teenagers; some devices even send a text message when the car goes over a certain speed.
Sales of GPS trackers to private individuals may have already surpassed more than 100,000 per year, some experts believe. The marketing is just getting started.http://www.nytimes.com/2012/01/29/us/gps-devices-are-being-used-to-track-cars-and-errant-spouses.html?_r=2&hpw