I seem to recall a SciFi book by Robert Heinlein in the 50's that talks about microwave beaming of power from an in-orbit solar-microwave power station.
Well there needs to be a lot of micro wave floating around to produce 7+ volts what's a cell phone 300 mw? receive side maybe .18 microvolts? Just find out where the cops are running a speed trap and charge your cell phone back up.
I have been reading about it in both Science Fiction and Science Fact for some time.. As someone said up thread Tesla first purposed a wireless power transmission system.
For low power needs, given the amount of electronic pollution we are exposed to.. It can work quite well.