Check out this offering from APC

http://apc.com/products/family/index.cfm?id=314&tsk=x290x

Availability and productivity on the go.
APC Universal Mobile Device Batteries provide your mobile devices with extra power when you need it most. They can be used as emergency batteries, or simply as a way to get extra runtime so you can leave your heavy charging adapters home. These external battery packs supply power via a USB port to any mobile device that can be powered using a USB charging cable (sold separately), like mobile phones, iPods, PDAs and more. They are also small and light enough to carry in your pocket. Now you never have to be stranded without the battery power to make a call on your mobile phone or retrieve data from your PDA. Choose from rechargeable solutions or one that uses common "AAA" batteries.