We don't know if your iPhone 4's battery life will really be doubled by the Exolife battery case, but at least we know that the case is slim, light, and will give your gadget a bit of a power boost.
The case weighs a reasonable 59 grams including the rechargeable Lithium polymer battery. It has a polycarbonate frame and an Illuminated power meter. It'll run you about $90 and should be available at the end of the month. [Exogear]