There are cases and then there are functional cases. The functional variety don't just protect your iPhone, but go beyond by adding extra utility. Granted, we're expecting the upcoming iPhone 5 to have a larger screen, but if you're a current 4/4S user, these cases could extend the utility and life of your current iOS device until the 5S...
1. FreedomPop Sleeve: Tired of your carrier's lame 3G coverage? Then slip on this sleeve from FreedomPop, which hijacks your 3G signal and uses FreedomPop's own, which is supposed to be 15 times faster. The bonus with using FreedomPop is that there is no contract, so you can cancel it anytime.
2. V-Moda VAMP Headphone Amp & Digital to Analog Converter: This $650 case is perfect for audiophiles on the go who want to use their premium headphones while traveling. The case comes with a headphone amp and digital to analog converter, which is basically what you need at home to get decent sound into your cans.
3. Mophie Juice Pack Plus: Making your iPhone slightly thicker and bigger, the Mophie Juice Pack Plus is a convenient external battery case to have whenever you start running out of power. The 2,000 mAh battery more than doubles the battery life of your iPhone.
4. iFuzen Headphone Amp Case: If you're unwilling to spend $650 on a headphone amp case, then you should check out the iFuzen HP-1#. This is a revision of their older model, the iFuzen HP-1. It's a lot cheaper and lacks a digital to analog converter, but it will probably work pretty well.
5. iGills Underwater Diving Case: It's always worrying when your iPhone gets wet, but once you put this case on, you'll be ready to stream your life whilst diving. Technically speaking, you won't get any reception underwater, but you'll still be able to snap pics of nearby marine life.
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(Images: as linked above)