Anyone playing with non-standard charging should read this:

http://sw.nokia.com/id/3378ff2b-4016...on_v1_2_en.pdf

Anyways, the standard charger does not give 5V, it's more like 8V or so, less under load. Voltage should never exceed 9.3V according to that spec, in the N800, you'll risk tripping the built-in zener diode and then blowing the 2A fuse. I guess if a solar panel can't supply more than 500mA, it should be safe, just make sure there isn't a capacitor in line that could supply a short-duration high-current spike.

If the input voltage is around 5V, the N800 will see it as a "special" charger and never actually fully charge the battery. AFAIK, LiPol batteries are not very efficient to charge the last 20%, so it's better to just keep it at that level unless you have plenty cheap power available (that is, a wall charger), and can afford to turn most of it into heat.