We've looked at a LOT of external iPhone battery solutions here at iSmashPhone.com: tethered external bricks, sync-port plugins/hang-off-froms, even batteries built into cases. The bricks are high-capacity but inelegant: you have to carry the battery AND a sync cord at all times. The plugins are more compact, but lower capacity, and sometimes get in the way of regular phone usage. Similarly, the case/battery options offer one-stop-shopping convenience, but at the cost of having to set aside any cases/storage solutions you've already invested in.
Third Rail offers a novel twist on the case/battery option: a modular case and external battery pack system that can be used separately or together. Start with a slim hardshell case that fits both AT&T and Verizon iPhone 4s. By itself, the case adds minimal bulk or weight to your phone. Now, if you need a shot of juice, snap a Smart Battery onto the back of the case, and you now have an additional 1250 mAh of power that adds less than an ounce 'n' a half to the total weight. (Don't worry about having to power down your iPhone; the battery can be hot-swapped.) The Smart Battery also doubles as a non-device-specific external battery that'll work even when attached to the iPhone's case. And since the battery's not hard-wired to the case, you power users can get extra Smart Batteries and swap 'em in and out as needed.
The case itself retails for US$39.99. Each Smart Battery will set you back US$59.99. A case/battery combo set at US$89.99 saves you ten bucks off the a la carte prices.
Check out our hands-on review below: