Herrman calls this Case-Mate iPhone 3G/3GS case "high stakes iPhoning", and he's right. This is saying to potential muggers, "I'm going all in. If you want it all, just take my phone."
There's no denying the convenience of putting all your stuff into one device when you want to travel light. Potential ID-flinging when you take the phone out of your pocket aside, it's definitely an interesting idea, and a physical interpretation of the "smart phone" that keeps all your payment and identification information on it.
It is only $US30, and it does protect your phone with a protective film, which makes it at least as good as regular cases. So if you're the kind of person who has a money clip instead of a wallet, pry off $US30 and send it to Case-Mate. [Case-Mate]