For heavy users an additional battery is a must, but these at the moment take the form of an outer case or spare charger battery all of which are inherently inefficient because the internal battery of the iphone (about 1500mAh) is 3.7v, it is charged via a 5V source. The additional battery is also 3.7V and thus needs a circuit to "boost" the voltage to 5V to get the iPhone to accept to charge. Both the booster circuit, to 5V, and the charging circuit, to 4.2v, generate heat....wasting what is already in short supply. Here's what I think people could manufacture, a replacement back that is thicker than the normal one but that would house an additional battery, perhaps of identical amperage to the stock battery. The two then can be joined INSIDE the case thus creating a new slightly thicker and heavier iphone but with a doubling in battery capacity. This approach will be THINNER and LIGHTER than any slip on case of the same capacity and yet is much more efficient electrically, there is no wastage in the form of heat during boosting and reducing of voltage to charge. Think of it like if Apple were to sell a 3000mA version of the iPhone4 for "power user", those people will accept a thicker and heavier phone, much as they did when buying those accessory battery jacket, but much more efficient. Anyone seen anything like that?