Is it possible to build an energy harvesting device around cell phone frequencies?

Is it possible to build an energy harvesting device around cell phone frequencies?
I live in an area with lot of cell phone towers and many WiFi networks, I think it would be nice if I could energy harvest from them. How would I do that?

The first step in any energy-harvesting project, is to determine how much energy is available to be harvested. Look up how receiving antennas work, and the signal strengths these antennas receive. Calculate the power coming out of the antenna. Is this anywhere near a useful amount? Answering this question is called by some, determining the "entitlement" of the project.
Wirelessly-powered audio devices have been around for over a century: crystal radios. Look at the size of antenna a crystal radio needs to receive a strong AM broadcast station with useable volume.