We had a chance to take a close look at the Eton Mobius battery case for iPhone 4 and iPhone 4S here at CES 2012. This case not only features an 1800 mAh battery backup system but a solar charging panel on the back as well. Set your phone face down to gain a solar charge.
While Mophie and Incipio also make battery backup cases, Eton is unique in how they designed the case. Instead of having to force your phone into a slide case which may cause damage to the dock, the Mobius case slides to allow easy placement. Once your phone is in the case, snap the dock connector in place.
The Eton Mobius is currently available for $80 USD.