The main problem is that you have to trust the person who minted the coin not to extract the value of the coin while it is in circulation. Eventually it should

The main problem is that you have to trust the person who minted the coin not to extract the value of the coin while it is in circulation. Eventually it should leave circulation, be converted back to virtual, and transferred to a safe address that minter doesn't have access to. So, the thing that it is printed on should be really cheap, like card stock or just paper. It shouldn't be considered currency, but more like a check. People accept a check on the assumption it won't bounce because they know the person who signed the check to them. When you get one, it should be off the person who wrote the check, and you should cash it immediately (transfer it to an address you trust). I could see keeping a few business cards in my real wallet with public and private keys on them with empty addresses. I can load them on demand from my cell phone, and write the amount and sign them when I hand them to someone. This way I can give them money without them needing a digital device handy. I could even do this in advance in a few denominations for completely offline access, but it would still require the trust of two individuals on the same level as writing them a check. In this case, obscuring the private key would be pointless since both people would have access to it. Here is a good resource for printing address: https://www.bitaddress.org https://www.bitaddress.org It has QR codes on it, so you can use the Android app to scan the pub key and transfer money to it. The addresses are generated client side with your browser, so they should be safe from snooping.