A lot of cell phones now contain locator technology, either in the form of a GPS transceiver, or a system based on triangulating cell towers.
Some parents are using this to track their teenager's location at all times.
"Before, they might not have told me the truth, but now I know they're going to," said Mr. Lutz, 46, a lawyer who has been particularly protective of Britney and her sister, Chelsea, 17, since his wife died several years ago. "They know I care. And they know I'm watching."
(from http://nytimes.com/2003/12/21/technology/21WATC.html )
Would you do that?