Power Spring 4: First (?) iPhone 4 "Battery Case"
The MiLi Power Spring 4 claims to be both the thinnest and first external battery for the iPhone 4. Of course, first is debatable, because even at Amazon.com you can find other external battery cases. However, MiLi is well-known for prior products, so that's a plus, in its case.
The PowerSpring 4 has a 1600 mAh capacity (Amazon.com says 1200 in its product title, but if you look in the details, it says 1600). Pricing at Amazon.com is currently $84.58, but the site says there's a 1 to 2 month delay (this will hopefully change soon). MiLi is selling it directly for $79.95, however. We've always been fans of Amazon.com for newly released items due to their liberal return policy, though
These types of things have worked well in the past for other iPhone models. Some of them don't do pass through charging, either well or at all, however (sometimes you'll get one of those famous "charging is not supported" errors no matter what the manufacturer says. Thus, it's always better to stick with a name brand for these things.
Of course, this could all be avoided with swappable batteries. These things do add to the bulk of the device, after all. Apple has always said they can get better battery life with non-swappable batteries, but it's also created the issue that eventually the batteries need to be replaced, that that involves cost, time and more money than a simple battery.