Well, I know what I want for Christmas!
I actually imagined this board a long time ago, and spent many hours brainstorming how it could be built. I imagined a single LCD or LED screen with fiber-optic lenses to carry the changing keycaps through to the surface without interfering with the key action. I had a lot of uses for such a beast if only I could figure out how to make one. Too bad it was just technically unfeasible.
Fortunately, materials science moves on! The Optimus Keyboard by Art Lebedev will be both simpler in mechanical design, and amazingly progressive in its electronic design. The keycaps will each carry their own “Organic LED” display panel, allowing digital keycaps to be downloaded for each key.