One almost existed. The program I use to write TeX has several "keyboards". For example, typing Ctrl-g switches to the Greek "keyboard" and then "a" puts "\alpha" in your file; typing Ctrl-s and then "a" puts "\angle" in your file. At one time, the owners of the program were planning to produce a keyboard with the property that the symbol on the keyboard would change when you switched keyboards. So when you typed Ctrl-g, you would see $\alpha$ on the "a" key (I think they were planning to use LEDs). It was certainly an interesting idea, but, no, I wouldn't buy such a thing --- like most people here, I touch-type, and prefer to enter everything with a sequence of normal characters.