Those DIY'ers out there will appreciate the instructions laid out for creating your own "Solar Bag" from scratch at Talk2myshirt.com.
This is a basic bag that producs predictable results. The more sun the bag is exposed to the better it charges up small portables like your Ipod. Walk through a tree-laden shady park and you're out of luck.
The authors are going to come up with Version 2 of this bag that will include an on board battery in a later installment. But, this is a good starting point for beginners and a way to get some free supplemental power.