* Using wifi in a movie theatre to build a Google Analytics equivalent for movie studios. Total heartbeat and respiration data for every individual "user session" across all screenings. Devices like Roku could do this too and sell the data to Netflix.
* Amazon Echo could notice your heartrate is becoming elevated due to the start of flu - Amazon stages your go-to remedies in the nearest DC and pushes appropriate ads on the site.
* In store movement tracking. Why bother with phone beacons and hassle-some devices randomising their MAC addresses when you could just power up a wifi blanket and track bodies with precision.
I guess the lab setting of the above research wouldn't apply to these situations - but I imagine with more research and some specialised equipment a movie theatre could be made conducive to this kind of tracking? Would there be legal implications? Wifi is everywhere already...
Or allow studios to block playback if more than x people are present, unless you fork over for a "broadcast" license...
And, to keep it out of the public sphere it has to be kept out of the private sphere lest people get used to the ideas and let it leak into the public sphere without protest.
I was just intrigued by the perceptions around this - people often have an instinctive strongly negative response to the idea of pervasive face recognition but I'm not sure they would feel as unnerved by the presence of a wifi network with this sort of "surveillance" traffic running through it. Maybe they would.
Let me give you an example: technology like glass has made possible to sit behind people homes and look inside. You can even automate it with cameras. This is not done because there is no technology to allow it but because it is felt unethical.
In addition general population ignorance in something does not mean that more qualified people can not propose measures against unethical actions of small group of individuals. These measures can be then formed into another invention called law and applied by another invention called law enforcement.
Edit: or notice more carefully important marker. I am sorry about my comment. I was wrong.
Obligatory XKCD: http://xkcd.com/538/
The main consideration is that it largely depends on who is your attacker. If you think about regular people, you should be fine; state-sponsored attackers know way too many techniques for you to be safe, unless you get very paranoid. Also keep in mind that these attacks (AFAIK) do not scale and are not automated, so a state attacker should be explicitly targeting you.
Finally, you should consider that modern computers and mobile phones have so many security issues that using these advanced physical-world attacks is not necessary.
The only relevant attack could be to train on a specific target (person+laptop+location) for a given amount of time to achieve high accuracy, but to do this you already have to know what the person is typing...
If you have enough control over the environment and enough information about the subject to make this effective, wouldn't it be easier to simply hide a camera in the room? That said, it's a really cool idea!!! I wonder what kind of other data could be pulled from a laptop by looking at it's EM emissions.
I wonder whether you could use the same technique to implement a Leap-Motion style controller without any specialized motion tracking hardware.