Depending on your needs, you could also consider a large-capacity external Li-ion battery. A 20,000mAh model would charge your cell phone, iPod, and camera batteries at least twice each before it was out of juice, all for less money, less bulk, and less hassle than a solar panel. Plus, you can use it on plane flights, car trips, rainy days, and other situations where a solar panel would be useless.
Amazon has a bunch of cheapie models that get horrible reviews, but this
$80 model seems to be a crowd favorite. If you go the battery route, buy more amperage than you need, because some will be lost to heat generation, battery discharge, and manufacturers' exaggerated claims.
Alex