You could tell where your pet is at any moment within about thirty feet. In practice, however, there are several obstacles. First and foremost, the transceiver would have to be continuously powered, which means batteries, and heavy ones, at that. It would thus be too heavy for small pets to wear. Also, GPS requires line of site to the sky, and so won't work in most buildings.
Coverage is also spotty in some regions.
Finally, most U.S. cellular networks aren't optimized for sending data, including GPS latitude and longitude readings, so there could be problems conveying the data to the monitoring station. They stirred up a lot of interest, but according to an article in Wired , never shipped any product. LoJack works by installing a beacon in your car that the police activate via radio waves when you report that your car is missing. Once activated, the beacon sends a signal with the car's location information. We're also guessing that the beacons, being designed for cars and construction equipment, would be too heavy for most pets to carry around.