Eton has a lot of solar-powered products such as emergency gadgets like the Raptor and the Scorpion. Their most recent product, the Mobius, takes a page from the Mophie Juice Pack playbook as it is a portable charger specifically designed for the iPhone 4.
The Juice Pack is a battery that doubles as an iPhone case, and the Mobius is essentially the same thing, but solar-powered. According to the official Press Release, one hour of sun on the solar cells is sufficient enough for 25 minutes of talk time, 20 minutes of internet use, 35 minutes of video playback, and two hours of audio playback.
In addition the 1800 mAh rechargeable lithium ion battery pack, this extra battery power will be held until the user needs it. All it takes is a flip of a switch, and some extra juice gets squeezed to an iPhone 4. The unit also has a micro USB port which can also charge the unit with the included micro-USB cable, and it will even sync with iTunes.
I am not certain how durable the solar cells are, and I don’t want to drop it to find out. The rest of the case is hard yet light metal, and the unit weighs about 3.4 ounces.
You should be able to get the Eton Mobius for the iPhone 4 on Amazon now for $82.34.