The US Air Force Research Lab is currently developing a micro air vehicle (MAV) that can harvest energy from power lines, which could lead to significantly longer surveillance missions. The aircraft would even have the ability to transform into inconspicuous objects, like an "innocuous piece of trash hanging from the cable." This would be achieved using existing technologies for collapsible wings and "sliding skins" that would allow the fuselage to change shape.
Naturally, the notion of landing on a power line is riddled with challenges. For starters, the device could easily cause a discharge of sparks that would give away its position. Despite these challenges and the admission that the whole concept is "bizarre," the AFRL is planning on conducting test flights in 2008. [New Scientist]