It also has its own 30-pin connector so you can sync your phone while it's in the case, a feature that other similar cases don't have. The case charges via USB, and there's an LED indicator showing you how much power remains in the lithium polymer battery.
I'm an advocate of external iPhone batteries, and this case for the iPhone 4 is thin but protective, and also packs additional battery life to keep your phone juiced through the longest days. The Snap Battery Case ($60) is, as its name implies, a snap-on case for the iPhone 4. It protects the phone without obscuring its design; the sides are open but the antenna is protected. And the battery itself doesn't take up too much room, either.