In reply to The sharp end:
We've just bought a freeloader Isis, because it is a solar panel/battery pack in one, has 2 inbuilt chargers (iPhone and miniUsb) and a usb for an extra charging port...
The panel is quite small but can be strapped onto a rucksack and seems the least faffy option. However the company might be in administration and a lot of places are sold out.
From what I've read, very few will ever fully charge from solar alone, but are a useful topup if you need it.