I made a little AVR-powered LCD badge starting with a schematic and going all the way to home-etched board about a year and a half ago. Maybe I could get a chip with some more horsepower and write some code to make it into a project like this. Some have suggested using a regular USB key to move between your secure PC and your internet PC. I don't like the idea of using USB for this because of the possibility of infection:
http://www.tomshardware.com/news/usb-flash-virus-secure,6564.htmlIf you plug in your USB key to an infected PC connected to the internet, and it's infected, then you plug it into your "vault" PC, the secure PC could also be infected. If the virus author is clever enough, your private key could be compromised when you plug the key back into the internet PC. Why not do all of this without USB? I wonder if a serial interface could be devised that would allow you to send in JSON of a requested transaction (like {address, amount}), have you go physically push a button to confirm it on the device, and have it send back to you a transaction. It's basically the OP's idea, but much more paranoid. I have all the equipment to do this - maybe I'll start soon.