It's not so long ago that I wrote about 'SpouseTracker', an imaginary system that would allow you to type your partner's mobile phone number into a Web site in order to find their location in the real world.

Recently that fiction has become reality. Last week I saw a newspaper article about a system that allows parents to track their children's location via their mobile phones, while a recent Internet advert promised employers the ability to locate their sales staff by the same method: are they really stuck in traffic or are they stuck into a pint in the Fist & Merkin?

None of this requires any clever technology, since mobile phones have always been traceable by their proximity to base stations. All that's changed is the perceived market, i.e. there is one and it's probably worth a packet, especially where parents and employers are concerned.

So where do we go from here? There are several possible options. People might decide to switch off their phones to avoid having their position tracked, although this may not be enough with future phones that may use passive transponders to report their location even when the phone is off. And anyway, the majority of people now seem unable to live without their vibrating friend (I'm still talking about phones - pay attention).

Maybe law enforcement organisations will tap into the scheme, so that they can identify the demonstrators - or at least their phone numbers - on a march, for example. Maybe mobile phones will become the identity/'entitlement' cards so beloved of various home secretaries over the years. Carry one or lose out.

All speculation, of course, but the future is nearer than you might think. In fact it's already here, in another guise. New Scientist magazine has reported several times recently on new RFID (radio frequency identification) chips. These are passive devices that transmit their unique identity number when interrogated by an electromagnetic signal. They are being used by supermarkets to track particular products such as clothing and razor blades, in some cases even triggering cameras to take photographs of people who pick up these products.

The chips are supposed to be deactivated when they leave the shop, but there's no technical reason why they have to be. Their range is short at the moment, but that will change, as will their application. In the future they could potentially be used to locate, for example, anyone who bought a particular brand of trainers, or a particular type of ice-cream.

This is great for advertisers, who will bombard us with targeted, in-your-face marketing material based on information about our buying habits and current location. But it's probably not something you'd be too happy about if you're a purchaser of pile cream, pornography or Private Eye. And the chips are becoming smarter; larger versions are already able to calculate their position, measure various physical features around them and form networks to process information.

Barring a major legal block on such devices (and if CCTV cameras can pass the "nothing to fear if you're a good citizen" test then so will RFID tags), these miniaturised tracking devices are here to stay and could end up as permanent features of our clothing, our shoes, our environment and maybe even our food. If you resent the intrusion of government and corporate interests in your life now, just wait until these little gadgets become microscopic and ubiquitous. At that point, true privacy as we know it would become a thing of the past.

This sounds like scare-mongering, but it's not. The technology already exists to do all of this. Whether or not such systems are actually implemented will depend on how people, including you, balance consumer desires against the need for privacy.