I've heard ideas for flying robots that would hook themselves onto power-lines like birds (or maybe more like bats I guess) and charge themselves with induction from the line.
Probably several kinds of illegal, but it would be pretty neat.
There are examples of wireless recharging [1], and UAVs perching on power lines for recharging [2]. Apologies for the self-indulgent links -- I wrote the articles, so they were easy for me to locate. ;-)
[1] http://www.hizook.com/blog/2012/04/17/wireless-power-transfe...
[2] http://www.hizook.com/blog/2010/07/20/gliding-uav-perches-po...
Then it could fly indefinitely.
I guess you'd need it to carry an AC/DC transformer though? Too heavy?