I was discussing the idea of devices recharging themselves from ambient radio waves (of which there are obviously plenty) with some friends a couple of years ago. I was generally looked at with doubtful eyes and there was plenty of chuckling going on.
A friend of mine just blogged about a company doing precisely this; Powercast. Granted they're not using ambient noise (yet), but they're doing it; wireless battery recharging from up to 30' away in some specialized cases. Too cool!