I'd give it 1 month on the market, max, before someone hacks a way to spoof the signal from a supercontroller device, giving anyone with internet access a way to control all the devices they want, if they so desire.
Cool, imagine this, I type something in my phone, all the phones in school start ringing and vibrating and nobody can turn them off because the keyboards are blocked, everybody panics and takes off his battery. I start laughing and end up in the principals office. This is the most innocent use of this technology that i can think off. Here's another one. Terrorists kill a train in the middle of a tunnel and blow up 2 bombs at the two ends sucking all the air and suffocating all the people to death. Of course if the people survive the extreme change in pressure, if they don't, they won't care that they have no air to breathe. Cool. O, and lets not forget police taking control of your car, making it drive you to a secret location and torture you and make you confess that you are a communist/terrorist/liberal/hippie or something. Some of us learned the lessons of totalitarianism the hard way you know.
I don't think you'd need the kill switch for the train scenario.
Agreed. Communications devices can't be distributed without also sharing their method of communication. This is why we all know the number 0x09f911029d74e35bd84156c5635688c0 as the key to decrypting HD-DVD movies.