If you use a Bluetooth headset, you know that, when not in use, it's just another little gadget to carry around and worry about losing. And that's no good. Which is why the Mogo Talk Bluetooth headset/case for the iPhone 4 makes sense: it lets you carry your headset right on the back of your phone when not in use.
Obviously, a case like this that can hold a headset will ad some heft to your phone, but it actually doesn't look too bad. It'll set you back $100, which is certainly more than most cases, but most cases don't come with a Bluetooth headset.