As much as I like the idea of an iPhone case that sports extra battery storage they tend to be too thick and bulky. It looks like the folks at Mophie agree and have come out with a thinner and more powerful version of their battery infused iPhone 4 & 4S Case. The Mophie Juice Pack Air comes in three colors, black, white, and red, yours for about $80 here.
With the juice pack air protecting and charging your iPhone, you can enjoy up to 36 extra hours of audio and nine extra hours of video playback, in addition to the battery life of your iPhone.
iPhone 4:
Standby Time: Up to 270 hours
Talk Time: Up to 6 hours on 3G | Up to 12 hours on 2G
Internet Use: Up to 5 hours on 3G | Up to 9 hours on Wi-Fi
Audio Playback: Up to 36 hours
Video Playback: Up to 9 hours