You can make a very simple GSM signal detector using a germanium diode, an LED, and a bit of wire. Tune the wire by cutting it to the right length (a square with sides of 7.5 cm is fine) and solder the diodes in series. Hold a phone near it and send a text message to the phone.

http://www.creative-science.org.uk/mobile_LED.html

Of course, that's a tiny amount of power but it's still there to be harvested.

Not sure what the legal situation is. In England an "artist" was asked to remove their installation of flourescent tubes in a field below power lines. They similarly harvested power. (I can't find that case using Google :-( but here are plenty of others https://www.google.co.uk/search?q=fluorescent+tubes+power+li... )