Proporta (http://www.proporta.com) has released the TurboCharger Back Pack, a rechargeable battery pack for the iPhone 4 that charges from any standard USB port and stores 1700 mAh. It costs approximately US$68.
Insert the iPhone 4 into the Back Pack by separating the two sections and then sliding them back together once the iPhone is in place. The Back Pack will automatically commence charging and you’ll see the charging icon appear on your iPhone.
An LED display shows the level of charge. If you connect the Back Pack with your iPhone housed you can still charge the Back Pack using the USB cable provided. This will also sync your iPhone with your computer.