On my last tours I used a Civilian Lab Tanker (TM) USB Power Bank. It charges and discharges 4x2300 MAh AA NiMhbatteries, It charges like a wall-wart and it has a USB port that I used to power my Android phone between charges.
The AA batteries were also used to operate my Steripen and my digital camera before it went 10-toes-up in Colorado, keeping me from having to carry spares for it all. I liked the versatility of this unit.
I have not checked to see if it will power my iPhone 4, but I'll do that before my next tour.
http://www.amazon.com/Civilian-Lab-T.../dp/B000QUN63C