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.
I totally second Alex's suggestion. I have a 13W solar panel which provided one single charge to my cell phone during a week in the Broken Group Islands. I have since bought a 20,000mAH battery pack (Costco - $40) and will henceforth leave the solar panel at home. It will provide about 4 full charges to my phone and I have even used it to charge my tablet.
If you do go the Solar panel route, you can't go wrong with Graham Morfit. First class guy, with a first class business.