If you're prepared to spend at least $200, I highly recommend looking at the Brunton Solar Rolls. These are flexible, waterproof solar panels with lash points available in different sizes depending on your power needs. Personally, I use the Solar Roll 4.5 which measures 12" by 22" when unrolled weighs less than half a pound, straps nicely across the top of of whatever I'm carrying on my rear rack and charges my iPod in 3 or 4 hours, and my cell phone in a bit less, depending on the weather. These panels are expedition quality, hence the price.
There are less expensive options out there ($40 - $100) specifically designed to be portable and used for cell phones and other small portable electronic devices but these panels are generally not waterproof, rigid, and require slightly longer charge times.
[edit]It seems I should have checked the Brunton website myself before writing this reply. The company now offers a 6 watt, 7oz, iPod specific charger for about $80 less than the version I mentioned above though this version is folds instead of rolls and, if I had to guess, is not as durable nor as weather proof. [/edit]
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i'd rather be biking.
This message was edited by tofubicycle on 3-12-07 @ 7:08 PM