The suggestion of harvesting energy, electricity, has been lingering for over a century ---ever since Nikola Tesla drew up plans to transmit electricity from large towers around the globe without using any connective infrastructure--- and still, today, it still may strike us as as a bit of a stretch. However, it's clear that as each of us become more dependent on small devices that require some source of energy(read: mobile phones, e-readers, laptops, mp3 players, digital cameras), demand for ambient energy will continue to grow. If you've ever found yourself waiting to use an outlet in an airport terminal behind a queue of three other equally desperate travelers, you'll know what I'm talking about.
There is good news for those of us dependent on our gadgets though. A number of companies seem to be working towards a practical systems solution to this old but not yet implemented idea; these companies which include Fulton Innovation, eCoupled, WiTricity and Powercast are working towards solutions that would deliver energy over relatively short distances. Eventually, we may be able to power our personal devices by simply using a dedicated microwave transmitter---our devices would pick up the microwaves with an antenna and convert the signal into electricity; such technology has already been tested on a number of occasions during which sensors were powered by energy collected through a microwave transmitter.
Of course ambient energy does present a few challenges, the principal of which is how does one charge consumers for something they get from the air? Indeed, metering ambient energy presents a bit of a dilemma. I wouldn't worry though, I'm sure inventors will come up with a way to charge us for this service.
If you're interested in learning more about these developments, have a look at this article which ran in the Economist two weeks ago.