Newegg is having a sale on a Energizer protective case that has a built in battery for your iphone on sale for $39.99. The iphone will fit into the case and it will extend the battery life of your iphone. It retails for $79.99 

Pass-Through USB: Charge and sync your iPhone 3G and G3 S to iTunes without removing it from the iPower2 via the included USB cable

• Charging: You can use the included USB cable to charge the iPower2 and iPhone simultaneously or just charge the iPower2.
• Intelligent Power Distribution: Smart Battery Technology instructs the iPhone to always drain out the iPower2 first. When you are done with the iPower2, simply detach it and you still have a fully charged iPhone.
• Safety First: The iPower2 utilizes advanced rechargeable lithium-polymer battery cell technology for added safety. Built-in circuitry prevents the iPowre2 from shorting, over-charging or malfunctioning even under the most extreme conditions.
• LED Battery Status Indicator: The integrated LED status indicator tells you how much power is left in your iPower. The LED lights also indicate the charging status and
• charging progress.