The Mooncharge iPhone case shown in the images above and below is a hybrid solar/battery case for iPhone 4’s that includes a rechargeable lithium-ion battery that can be charged via a built in solar panel or through USB.
The Mooncharge Hybrid Solar Battery Case allows you to power your iPhone for an extra 315 hours on standby, 27 hours of music, 5 hours of talk time, or 10 hours of video. That is, when the battery within the case is fully charged.
To put this in perspective, 20 minutes of direct sunlight will buy you an extra 50 minutes on standby, or 5 minutes of talk time. However, the battery can also be charged via USB, allowing you to essentially use it as a back up battery.
There has been talk of Apple including some sort of solar technology natively into the iPhone or other iDevices. Until then, you can grab the Mooncharge case from Frostfire for $70.