Ralph Lauren's recently released Solar Panel Backpack is a black or bright orange water-resistant bag that provides a useful 3.45 watts of power. According to the luxury label, this would enable iPhone users to achieve a full charge in less than three hours--on a sunny day of course. Our own rough calculations for an iPhone 4 seem to verify that figure, but your own mileage may vary.
The official Ralph Lauren site also vaguely mentions compatibility with the iPod Classic and "mobile phone devices," so users of non-Apple products might want to check if their device is supported.
While solar-powered accessories may appeal only to outdoorsy types and probably work best if you live in the tropics, any product that helps save fossil-fuel while charging our gadgets is definitely a step in the right direction.
The Solar Panel Backpack is available online for $795.