PEAR

I imagine this to be a small device the size of a pager or watch, strapped to the owner's wrist. Ordinarily, the device simply monitors the owner's heart-beat, respiration and key bodily functions and alerts the owner to any abnormal or dangerous levels. But it has other functions.

The device contains a small chip with the following data: name, ID, age, nationality, blood-type, allergies, special conditions (e.g. asthma). The owner can choose to add more data that might be useful. The data is normally encrypted but can be released by the owner; it will also release the data when the owner is unconscious or when his health is in peril. It will then transmit this data (using milli-wave, blue-tooth, whatever), together with the owner's current health condition, to special receivers issued to the police force, medics and emergency rescuers.

During emergencies, the device will even broadcast a distress signal to all police stations and hospitals in the vicinity. If the owner is buried in an earthquake or avalanche, the device will produce a strong signal to attract rescuers to the owner's location.

The device also contains: built-in torch, mini-compass. It also contains a panic button; upon activation, a small transmitter will scream "Help!" in any of 100 different languages (or rotating through various languages). And yes, able to transmit SOS in light, sound and smoke signals.

In case the peril is due to muggers, robbers or murderers, the device will also record incriminating evidence with its mini-camera and voice recorder.

Then of course, the device should be tamper-proof and ultra-durable, and issued to everyone from birth. It should be calibrated to the owner's physical profile, so if anyone else tries to steal it the device will know and broadcast an alert to the police.