Powercast (formerly Firefly Power Technologies, and spin-off based on University of Pittsburgh research) made a splash at CES this year with their dime-sized receiver that harvests RF energy from a nearby wall-wart transmitter. Based on their patent and related tech from PITT, the technology looks pretty darned simple (so simple I’m surprised there’s not prior art, but then this isn’t my field). It’s basically just an antenna with a bunch of taps, each tap consisting of an inductor to resonate with the desired RF frequency and a rectifying diode to turn the energy into DC. That DC voltage is integrated across a series of capacitors, and stored in another capacitor.
I’ve not seen any detailed specs on how efficiency drops off with range from the transmitter, though a Businesses 2.0 write-up claims their range is only about 3 feet, with voltages too small for laptops but good enough for small devices. Their tech has also been tested for recharging wireless sensors at the Pittsburgh Zoo, and Philips is apparently coming out with a wirelessly-powered lightstick using the technology later this year.