Continous authenticating Armband

I have always wondered what is the most secure and convinient way of authentication. Basically requirement should be that no one should be able to steal it, and it should be accessible to you always.
Was thinking of how bluetooth devices and automatc discovery works and the idea came. Long time back we had come up with a business idea where we implant a chip in a persons body and it continually monitors some vital stats and emits a signal. As long as ur alive, the signal is live, u die, signal changes. Going a bit further, the position of the signal can be tracked, so it works like a honing device. Good for armed forces, people into adventures, explorers et al. Am sure this must be in some Bond movie already. Anyways...
The idea now is that instead of a chip implant u wear an armband, which monitors ur vitals, not just to see if ur alive, but also for stress, like a polygraph. In turn, if the armband thinks ur normal, it emits a signal like in Bluetooth, and ur automatically logged in on devices in range.
Imagine the potential, the armband can be configured to hold ur identity. So no more passowrds for anything. Also the armband can be configured to log into services, devices of ur choice. As soon as u walk into work, ur work machine identifies u and logs u in. Ur phone is always locked, unless in range of ur armband, which is basically u. Walking into a shopping mall, the armband emits ur identity, which in turn is captured by the services in the mall and accordingly ur served with advert et al.
On the security side, since this is monitoring ur vitals, there is no question of theft or breaking under pressure, because it's basically connected to ur subconscious.
And the armband will have to be self learning, so it will start differentiating between stress levels, and will recognize ur moods.
A couple of problems...subconscious can be manipulated under the effects of medication.
The armband itself can be tampered with, or in worst case stolen, in which case u basically loose ur identity, no one can steal it, but u don't have it either.
Also am wondering if there should be a manual override, well gut feeling tells me for all practical purposes there needs to be (because the armband learning program can never be faultless), but that might cause a lot of problems, because a manual override can be misused one, it leaves the armband vulnerable.
Maybe for every armband there is a period of learning before u give full control of ur identity to the armband!!!

PS: another advantage would be airport security, the airport security systems will be configured to read the vital stats as well, and so if ur a terrorist with a bomb, they'll pick up the heightened stress level...no more full nude x-ray pics of u required. Am in a flight and just though of this!! might not work for secret agents trained to beat the polygraph though :-)