I had two 1.93W (8V @ 240mA max) unused solar panels, and decided to put them to use. Especially since we're blessed with lots of sunlight here in Norway these days, which is unusual.
The panels were wired in parallel to increase the amps, and connected to a USB female connector via a 7805 voltage regulator. Voltage dividers were added to the D- and D+ lines so that the iPhone will recognize it as a charger:
I wired it up last night and, to my surprise, the phone started charging even when the panel was held under the fluorescent kitchen lamp.
Today, in direct sunlight, this 1st gen iPhone with its 1200mAh battery was charged from 0% to 100% after approximately 2 hours. It's worth mentioning that the phone was switched on, and would probably charge faster if it was off.
The 7805 got very hot, so I found a heatsink and let it pass some of its heat through it. Gave a nice tilt to the device, so I don't care if it looks silly.