Full disclosure: I don't yet have any boom-box sort of portable speakers for any of my various iPods, mainly because I have been too lazy to research them. But I keep stumbling across mentions of a new solar-powered portable iPod speaker set, which is being sold by Frontgate.
At around $200, the Soulra Solar Powered iPod Speaker isn't exactly light on the wallet. But if you're someone who likes the great outdoors, in locations far beyond the reach of an extension cord, this gadget might be worth a look. The 8-wattt RMS speakers draw their power supply from the solar panel, delivering approximately 7 hours of play time per full charge. It is billed as water-resistant and has a bass boost if you are inclined that way.
Hammacher Schlemmer is also selling a solar-powered speaker dock. It requires three hours of solar charging per one hour of playage, but this one also comes with and AC adapter.