(FORTUNE Magazine) – I'm feeling watched. Like someone is following my every movement. Like electronic sensors are measuring my heart rate and body temperature, and beaming them over the Internet. Hey, maybe it's because of the tracking device I'm wearing on my wrist.

It's called Digital Angel, and by affixing it to my person, I'm in the vanguard of a revolution that will either save thousands of lives or destroy privacy and bring about the end of time. Depends on whom you ask.

When Digital Angel hits the market this fall ($299 plus a monthly service fee), it is supposed to do for people what LoJack did for cars: help monitor the whereabouts and vital signs of children, the elderly, medical patients, and even kidnapping targets. It's like a guardian angel watching over those at risk, says Richard Sullivan, the granite-faced CEO of Digital Angel's parent company, Applied Digital Solutions (Nasdaq: ADSX). "The benefits to mankind are enormous."

Crammed into the watchlike device on my wrist and a pagerlike gizmo on my hip, I'm told, are three technologies: a miniaturized GPS transceiver that lets the watch "know" its location; biosensors that collect data from my skin surface; and an antenna that transmits the information over AT&T's wireless network. Were I to give a password to, say, my mom, she could log on to the Internet and see that my heart is beating 83 times per minute (I've been walking briskly), that my temperature is a reassuring 98.3 degrees, and that my latitude and longitude (26.70577 degrees north and 80.04111 degrees west, respectively) place me on a palmy strip of road in Palm Beach, Fla., just outside Applied Digital Solutions' offices. Assuming, of course, I haven't taken off the watch to give her the slip.

The system promises to only get better--or creepier--from here. Executives say that future versions will measure glucose, alcohol, and oxygen in the bloodstream, instantly notifying Digital Angel's 24-hour center of doctors if the levels cross certain thresholds. The hip unit will disappear, and instead of relying on a battery the device will be powered by body heat.

And it could be embedded in the skin--which is where the end-of-time part comes in.

As late as last summer, the company was planning to make its gadget implantable, having acquired a patent to an "apparatus for tracking and recovering humans utiliz[ing] an implantable transceiver...small enough to be implanted in a child." As if to audition for a part in Austin Powers III, it acquired the spookily named Destron Fearing Corp. (say it fast; it sounds like "Death Star Fearing"), which was already selling implantable identity chips for pets, and hired a mysterious scientific genius named Dr. Zhou to make it all work.

The plan, quelle surprise, blew up faster than you can say "New World Order." Privacy advocates freaked. E-mail poured in. Christian evangelicals declared it the Mark of the Beast--a reference to Revelations 13 ("[And the Beast] causes all...to be marked on the right hand or the forehead, so that no one can buy or sell unless he has the mark") and a sign of the apocalypse.

"We were totally blind-sided," says Keith Bolton, the company's chief technologist. Plainly uncomfortable with being cast as the Antichrist, he asks, "How could anything that brings such hope to people be the entry to an evil time?" The company has since toned down its messianic rhetoric (no more pontificating that people "will be a hybrid of electronic intelligence and our own soul") but has not forsworn the idea of future implantables--though don't call them that. "I think the term 'implanting' is actually not the correct word," says Sullivan. "The word is 'participating.'"

Either way, executives say they "anticipate a blooming fortune" from a market they estimate at $70 billion, noting that several thousand people have pre-registered for the product. "Digital Angel is going to be at the forefront" of what President Mercedes Walton calls "this whole emerging area of cyborgs," or the convergence of the biological and the digital. Equally, though, it may be about the collision of the commercial and the ethical.

On a Christian chat board full of comments like "digital angel is a spiritual cyanide capsule," and "isn't it obvious that this is a tactic of Satan!?!" a message of a slightly different flavor appears: "Is there going to be stocks on this product? If so, please tell where I could get a hold of it. Thank you!"

--Jerry Useem