I had a thought about doing something like this, but the arithmetic indicates its hard even to get 1mW out of it, even if you're in a room full of people on the phone or next to a radio station. I got the idea when 25 years ago I left the antenna I had put on my parents' roof connected to a field strength meter antenna input. (I've always been a nerd.) One day I noticed the meter deflecting. There was a strong signal around. On an ordinary day it only deflected a little due to all the ambient RF energy. I didn't have a spectra analyzer, so I never found the source of that signal. It got me thinking, though, the energy was enough to deflect a physical meter; could we harvest it?

I belive the answer is no unless you really need only microwatt. I love people trying. About a year ago I interviewed a professor who was working on energy harvesting in shoes. http://bit.ly/REWODe Energy harvesting is interesting, but I never see a way to get significant energy out of it.

lots of this kind of good stuff going on right now..