Lenmar Battery has recently come out with a cover for the iPhone 3G and iPhone 3Gs. The cover itself is a fairly standard hardened rubber sort of affair, with a couple of notable exceptions. For one it has a battery built into the case, which can be charged from an external USB source, it also addresses the lack of a flash on the iPhone.
Americans use around 2 Billion disposable batteries a year, one of those numbers that’s big enough for most of us to have no actual concept of it, but it’s a lot. If you find yourself being a larger than normal part of this statistic the obvious solution would be to use a rechargeable battery.