Etón Corp. has released the US$100 Mobius (http://macte.ch/ONVP6), a rechargeable battery case with a solar panel for the iPhone 4.
It features a quick charge time and the ability to store charge until you need it. The monocrystal solar panel re-charges the 1800 mAh rechargeable lithium ion battery pack on the back of the case. Only one hour of solar charging provides an additional 25 minutes of talk time, according to the folks at Etón.
The Mobius can also be charged via the included micro-USB cable and will sync with iTunes. It features a standby switch to turn off direct power transfer to the iPhone 4, allowing the case to charge and store energy in its own battery.