MacRumors recently picked up on a patent application from Apple that involves a potential future solar powered iPod.
The patent basically describes an iPod with a solar panel underneath the LCD screen that could be used to provide additional power or charging to the iPod. According to the source the size of the panel is too small though to be able to totally powering the iPod. Perhaps the next patent will be for a revolutionizing effective solar panel.
This follows in line with the japanese solar powered bra that can be used to charge an iPod.

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we use a 100W solar charger at home to charge lead acid batteries”‘*