This is what scientists from University of Washington and Intel intend to achieve in the near future - passive tags which would work on power derived from air. If I talk in near technical terms then these tags would derive RF power from several sources such as radio, television, cellphone base stations transmitting it.

In a recent experiment the prototype Wireless Ambient Radio Power (WARP) device was mounted on the balcony of the Seattle Lab and the antenna was aimed at the Seattle TV transmission tower located around 2.5 miles away and was able to tap the RF signals and pile up sufficient energy to power commercial hygrometer/ thermometer with LCD display. The prototype included power harvester comprising of printed circuit board wired to a set top television antenna.

The WARP device is a part of the Wireless Identification and Sensing Platform or WISP project undertaken for building passive RFID tags which can exploit RF energy for supporting sensing applications. Seems pretty interesting I must say!!