My first thought is that literally billions of people already do something similar every day. They use a card that runs its own small, programmable computer, th

My first thought is that literally billions of people already do something similar every day. They use a card that runs its own small, programmable computer, that is portable, energy efficient, and can be powered 'wirelessly.' It's called a universal integrated circuit card (UICC) -- or the chip used in credit cards / sim / ID cards. The OS these cards run is often referred to as the "Java Card Platform." The OS has many of the same features you would expect from a modern operating system like support for multiple programs, I/O, and very strong crypto primitives. It even has features you wouldn't expect (because some applets give you access to APIs that aren't currently open source available outside that environment.)