actually, a few days ago i saw a tv-report on useful gadgets, and they were testing a solar-charger. it was basically just like the official external charger form nokia, just that you dont load that on the wall, but by the sun. and even though it was cloudy, it was loading the battery with full 5V.
i guess its worth a try.

i think the other dead sure solution would be one of those devices you have to wind up to generate electricity. takes a lot of time though...^^