There are quite a few DIYers out there. One fellow built a solar charger for his iPhone. What’s more, he calls himself a DIY n00b, so it can’t be too hard to put together.
We just plug our iPhone into a computer or wall, but if you feel the need to be green, or just to do something for fun, check out the instructions for this solar powered iPhone charger. It only took him three hours to do it. Keep in mind it does cost a bit, probably more than a charging case would if you buy the stuff he specified. That said, part of the entertainment for do-it-yourselfers is just the fun of building it.
Full instructions at Ars Technica