Since a wireless charger (RCA Airnergy) was announces at CES that harvests energy from wifi signals that are around the topic is everywhere in the (tech) news. It is an exciting prospective to imagine mobile devices that will recharge themselves from the environment they are in. However I am not sure how well harvesting energy from RF will worked in regular environments – not saturated with radio signals.
The topic is really exciting – and in the end people probably do not care much if the energy is harvested from their movement, from light, temperature or RF – as long as it works. There is a introductory article to energy harvesting by Joe Paradiso and Thad Starner  which is worthwhile to read.
 Joseph A. Paradiso, Thad Starner, “Energy Scavenging for Mobile and Wireless Electronics,” IEEE Pervasive Computing, vol. 4, no. 1, pp. 18-27, Jan.-Mar. 2005, doi:10.1109/MPRV.2005.9