The Pi runs off USB which according to the spec is 4.75v - 5.25v and can draw up to 700ma.
Often specs are over rated



Anyway, I've now put together a solar charger suitable for the Pi. Take a look, and feel free to ask any questions, or go to the 'I want one!' section to buy one.
The panel is a 2 watt, 330ma panel. - I 've got a variety of other panels ranging from 200ma up to 1000ma that I could use, but this one seemed a nice match. I'm thinking of using a different shape one at similar current, as well as looking at putting together a 1000ma panel with 10,000ma D Cells

This should give really good runtimes while still being portable.
I really want to get it set up as a web server and perhaps even run this Forum using solar - don't see why it shouldn't be possible off a Pi.