This is somewhat NOT new. You know how the military always refers to wired communications as 'secure' from intercept. Those were not the case during WWI. Early morse type fuller phones used dual or single wires for transmission/reception. Single wire worked using earth return. The problem was that the Germans figured out they could use some not so fancy gear to more or less eavesdrop on fullerphone (http://home.planet.nl/~meuls003/fullerphone/fullerphone.html) conversations which were using earth return/ground spikes.
Basically, where they can get noticeable differences in voltages from your ground leg, they're going to be able to intercept. Power and Ground voltage filtering would help this greatly. I wonder if a capacitor attached to your ground lead would dampen the square waves that the keyboards send. Where the grounds aren't easy to get to, one could have a problem. But even a house has external faucets and outlets where one could run a tap line to.
I guess filtering the ground (you can't isolate it) at the power connection would help too.
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Basically, where they can get noticeable differences in voltages from your ground leg, they're going to be able to intercept. Power and Ground voltage filtering would help this greatly. I wonder if a capacitor attached to your ground lead would dampen the square waves that the keyboards send. Where the grounds aren't easy to get to, one could have a problem. But even a house has external faucets and outlets where one could run a tap line to.
I guess filtering the ground (you can't isolate it) at the power connection would help too.