Incase has released a new version of their popular Slider case for the iPhone with an integrated battery inside. Called the "Power Slider", the new case has built-in five LED power level status lights in the back (like the MacBook) and a pass-through port that allows you to charge and sync the phone without removing the case itself.
It obviously adds to the thickness of the phone more than a typical case would, but it's the first extended battery case that I'm actually considering picking up. And I hate cases. Yet it's $100, which is a painful premium to anticipate a situation which I rarely find myself in. (I work from home; I know that many iPhone 3G users who are out and about all day have battery life issues.)
Power Slider iPhone 3G case product page [GoIncase.com]