The Aigo aiPower i616 iPhone battery case claims to double the battery life of your iPhone. It features a 1,500mAh battery, as well as a USB ribbon cable built-in, allowing you to easily charge your phone from USB ports without having to worry about losing a separate cable.
That should come in handy. If you need a longer cable you can connect a standard microUSB cable into the port on the left side of the battery. It will cost you $80, but hey, you are doubling the battery life. Is it worth it? You decide.