Just as I was thinking about what it would take to make a mars rover with the NXT, I found the dSolar panels from Dexter Industries. How cool is that, a mars rover that actually runs on solar cells!
However, $100 seems a bit expensive for such a nice-to-have feature. Looking around on eBay revealed you can get more power for half the money, only it doesn’t come in a LEGO friendly package.
I bought them anyway, and documented the customizations. Soldering iron required!
[update]: It is recommended that you put diodes between the panels, see comments below.
So basically I connected the panels in parallel with a few plugs from an old computer. The whole thing is connects to the NXT via 2 fake batteries made of hot glue cartridges. To prove it really works:
I can’t wait to make a robot with these.