I have a STY 8800mAh battery pack from dealextreme with USB output and an adapter to connect to the Nokia n810 (or n800). It will charge it and almost anything else via the USB or the main output.

WARNING - the Nokias will indicate "Charging" if ANY current is going in to the port. I've even looped back the USB port (host mode) to the charger and it shows "charging". I didn't know the n810 was a perpetual motion device. I do have some solar chargers and such, and they probably will maintain or extend the use time, but you need something which provides high enough current to actually charge the battery, and I don't know how that would be indicated.