There's always been speculation that Apple would be adding a solar power option at some point. There've even been patents filed to that effect, but a company called Frostfire has beaten them to it.
Frostfire has designed the (drumroll, please) Mooncharge Hybrid Solar Battery Case For iPhone 4!
The case has a rechargeable lithium-ion battery with a small solar panel on it that can extend the battery life of your iDevice whilst on the go.
Twenty minutes of direct sunlight shining on the Mooncharge case will net an extra 50 minutes on standby mode, or 5 minutes of chatting. Not a whole lot granted, but it's certainly something in a pinch.
Top up your battery while on the move by placing the Mooncharge in direct sunlight. If you get short on power or need your phone for an emergency, simply place the case in the sun for 20 minutes for an extra 50 minutes on standby or 5 minutes of talk time. The case uses four LED indicators to let you know when you are getting low on battery.
Frostfire empowers you to take advantage of the power of the sun through this innovative solar technology for your iPhone. Using revolutionary technology there is no need to be caught without sufficient battery power for your iPhone, simply turn on your reserve power as you require more battery juice.
The Mooncharge case can also be charged via USB cord, will get you up to 315 hours on standby, or 5 extra hours of talk time.
There is also a 3GS version available, they retails for $70.