Back at Macworld, I checked out a company on the show floor that made solar (photovoltaic) panels. I was hoping to see a panel for sale ready to charge the iPhone, because an easy-to-use panel at a reasonable price would have probably had me opening my wallet on the spot. Unfortunately, they only made custom-designed panels, and nothing mass market (or in my price range), so I was disappointed. But this one might be what I was looking for: Quickertek, makers of all sorts of Mac-related accessories, has adapted its MacBook "Apple Juicz" solar panel to the iPad. This provides 10 watts of sun-based power (reportedly the same as the regular iPad charger) to your magical and revolutionary device.
I don't have an iPad yet, and the price of the Apple Juicz (US$250) is still a little steep for me, even though it's probably cheap for what it is. But I am always in favor of using solar technology, and if you're doing something like camping in the middle of nowhere and using your iPad or MacBook a lot, this might be a workable solution.
If nothing else, at least we're getting closer. Someday, photovoltaic panels will be so good at grabbing light and batteries will be so good at storing the generated power that all you'll need to do to charge your iPad is go read outside in the sun for a bit.