Loopt, Inc and Sprint Nextel (the cellular carrier) have announced a provocative new service that "scans" a 25-mile radius around you and supplies a real-time map of your buddies' locations. It's symmetrical, of course: If you can see someone, that someone can see you.
This isn't the first cellular monitoring system. Sprint Nextel, Verizon and Disney Mobile offer services for people who want to track (their) children. But it is (I can guarantee this) the first one I've noticed that's for general use. And it's fascinating, more from a social than a technical perspective. For example, I can imagine a subtle "dance of the blips:" My buddy's blip approaches my blip even as I move away in order to avoid him. (Maybe I'm not in the mood for company--but actually texting him to that effect might be rude.)
Another problem: What if I decide that I no longer want to be visible to buddy A--maybe he's been abusing his knowledge of my location by dominating my time. How do I explain? What's the gentle, face-saving formulation? ("I don't think we should see each other anymore..." seems no kinder for GPS breakups than it is for romantic ones.) Finally (and there was a stalking story along these lines a couple of years ago), the technology is essentially a cheap, DIY car monitoring system. It's easy: Buy two phones, link them, and glue one to the target car's undercarriage. Voila! Instant LoJack. It could also work for pets. (Not that you should be gluing phones to your cat's undercarriage...or, on the other hand, go for it--I hate cats.)
All in all, Loopt/Sprint's new service represents an exciting development in the continuing technology/privacy tango, and I for one am excited by the possibilities. In fact, let's both join--I promise I'll stay with you always.