It still seems to me that you can't get something for nothing. As more and more devices start parasitizing power from WiFi or other devices, that power will not be available for the intended purposes. At some point I'd predict that the reduction in range and coverage associated with these parasitic loads will become observable and annoying.
...Are we not inviting more trouble while trying to solve one problem? At least we did not have security threats from a simple light bulb. Now we have that threat with an "intelligent light bulb"...similarly, in order to get rid of the batteries we are going to depend on WiFi to power-up the smart sensors, thus making those vulnerable to threats at the same time. If someone breaks into the WiFi and makes it disabled, all those sensors dependent on the same would be crippled...possible?
"Scavenging experiments utilizing TV bands have already yielded power amounting to hundreds of microwatts, and multi-band systems are expected to generate one milliwatt or more. That amount of power is enough to operate many small electronic devices, including a variety of sensors and microprocessors."
There are also some interesting projects in Kickstarter, such as iFind/Wetag, though I don't understand their tricks clearly.
This idea of harvesting energy is very good for sensor netwroks, where the sensors are so remotely placed that distributing power to them via conventional power netwroks is expensive and batteries are not feasible because of the maintenance and recharge issues.
Today, our atmosphere is filled with all kind of RF signals - internet traffic on wi-FI routers, FM radio signals, TV signal traffic over various frequency bands and so on.
So a good and judicious harvesting of these signals to generate the necessary power of sensors that are in open atmosphere and away from the power distributions networks will be defintely beneficial.