Check out this concept from Knut Karlsen. It almost seems too simple and elegant. The project has met with mixed results, but everyone giving it a go agrees that this is a concept that needs to be developed and seen in the real world.
Basically the idea is to wrap rechargable batteries in felxible solar cells, so that when the battery finds itself exposed to the sun, the rays are used to juice up the battery. Click on through to read more and see the step-by-step photos.