LOL, new catchy phrase for muggers: "your bitcoin tattoo or life!?"
Actually, carrying around your private and public keys might not be a bad way to solve the "physical bitcoin" problem. You'd still have to get onto the network to buy something, but you wouldn't need any kind of client yourself. Providing a client would be the vendor's responsibility, which is a lot more reasonable than expecting everyone to buy some sort of thin client or smartphone to buy things.
Oh! And we could tattoo our public keys on our foreheads and our private keys on our right hands and use those to buy and sell things! (Sorry, "mark of the beast" reference. Had to be made.)Edit:
Any idea how credit card companies and banks prevent businesses from storing their customers' card numbers and security codes and using them later to tack on additional charges? That would be highly relevant to us if we're to start giving out our private keys everywhere.