http://us.kensington.com/html/15462.html

US$50

Features
Power and charge your iPhone or iPod
Extends play time up to 30 hours of music*; 6 hours of video*; 3 hours of talk**
Includes retractable USB charging cable to recharge battery pack
High-capacity, rechargeable lithium-ion polymer battery pack for increased energy efficiency
Small enough to keep connected to your iPhone or iPod
LED battery meter displays remaining power level
*iPod nano 3rd generation **iPhone
Specifications

Works with iPhone and any iPod with dock connector (excluding iPod 3rd generation)




Good for those people who always wants to change battery for iPhone. US$50 sound reasonable.