|Image courtesy of dexter industries|
"dSolar Systems from Dexter Industries allow you to power your Lego Mindstorms NXT robot using nothing but the sun! This 9V solar power system is compatible with Lego Mindstorms NXT creations and includes everything you need to get started. Easily attach the solar panel to any of your Mindstorms NXT creations."
Interesting stuff! Browse to their website and have a look. There are also some interesting projects and videos on their blog.