There are plenty of voluntary ways to use your GPS-enabled phone to locate your friends in real time: Loopt, Google Latitude, or a simple text message will usually do. But if you're more into the constant-surveillance kind of thing, go ahead and spring for AT&T's new FamilyMap service.

Add $10 a month (for two family members) or $15 (for five) onto your family plan and you can spy on see where each of your relations are, using either your own phone or AT&T's map-mashed Web service. It works with pretty much any AT&T phone, using A-GPS on devices like a Blackberry or iPhone or just good old fashioned triangulation on dumbphones.

Unfortunately for Joe Stalker, once you sign up a phone to be mapped, AT&T sends a message to that phone informing it of the follow--and keeps sending messages as reminders about once a month. (You can also set reminders to ping a phone every time its location is looked up by another family member.)

One possible upside to the family in-fighting that will surely result from opting into this plan: It lets you recover stolen phones, as long as said phones aren't roaming outside the AT&T network.

So how useful will this service be? Even on smartphones with A-GPS, location technology offers limited accuracy, and things only get worse when you try to triangulate a dumbphone. Look up your son or daughter, and you're just as liable to see them either at a friends house, or a half-mile away under a highway overpass. Not to mention that A-GPS is easily stifled by buildings and other large objects. Worst-case scenario: your kid or spouse buys a private, prepaid GoPhone to stop your snooping ways; prepaid phones aren't eligible for FamilyMap.