Sanyo eneloop Mobile USB Booster for iPhone, iPod, PSP, Cell Phones and All USB Powered Devices
The capacity on this battery is nothing short of amazing: 5000mAH.    Look at the standard battery capacities of the some other backup batteries for the iPhone and you'll be amazed at how this one stacks up.    I was recently on a vacation trip at a theme park where I knew I would be away from power outlets for the entire day.  I used my iPhone 3G extensively, and found that the eneloop pack was good for at least two, if not three full charges of the phone.  Not only that, but it charges the phone up FAST, 