I'm an advocate 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.
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.