A company called FrostFire has created a lithium-ion iPhone 4 case which let’s you charge the phone with the sunlight hence increasing the battery while you are on the go. The case has a solar panel embedded on it which can buy you “enough” battery. When I say enough, it means 20 minutes of sunlight charge will let you have another 50 minutes on standby which is quite a lot in case of an emergency.
While if you charge it with a USB it will give you an extra standby of 315 hours hence buying you a lot of backup battery.
And all of that in 70 bucks, which I believe is quite worth the price. There have been speculations about Apple adding this to the idevices since forever but I think an external case is a better bet. Apple officially adding this feature means, you have to buy a lot more price for this. I’m better off with $70