An iPhone case that's also a portable battery...
For the best of both worlds, how about a case that comes equipped with a portable battery:
mophie – USB charger-cases available for iPod Touch and all iPhone's. The larger edition is available in Cyan, Magenta, Yellow, and Black, while Air (shown) is available in Red, White, or Black. Fitted volume and mute buttons on the iPhone 4. Smallest form factor is the “juice pack air" at 5.07" by 2.51" by 0.68" at 2.5 oz, capable of 1500 mAH. Price ($59.95 Sale) $79.95.
Boost Case – USB charger-cases available for the iPhone 3/3GS, 4, the iPad, and iPod Touch. Four blue LED indicators show battery and charge level. Like the Ford Model T, it's available in all the colors of matte black. The Boost Case for iPhone 4 is 5" by 2.4" by 0.69" at 2.24 oz, capable of 1700 mAh. The Boost Case's open fit for the side volume and mute buttons mean that it will fit the new Verizon iPhone 4, whose mute switch is shifted ever so slightly. Price $59.95.