Most iPhone battery cases have the same general design: a rigid plastic body with a slider component to insert and remove the phone. PowerSkin's PowerSkin for iPhone 4 ($50) is a bit different, though. It maintains a rigid back to house the battery, but the top and edges are a soft silicone rubber, pliable enough to allow the device to be inserted without taking the case apart. The case more than doubles the thickness of the iPhone 4, adding about…
