OK, I know I am harping on an old song, but I still believe in the idea: A capacitive touch screen with silicon markers for where the keys are. A touch screen is really quite fast, as long as you look at the keys. The problem is that as soon as you take your eyes off the keys, you have no way of knowing where you are. By adding a thin silicon grid or other overlay, you will get enough feedback to keep your fingers in place.

You can easily test what I mean on the TOHKBD or any other mobile phone keyboard; just imagine that the key would react when you just touch the key, rather than having to push it down. Do some pretend typing. Isn't that fast? Even faster than having to push the keys?

And of course you can change between any keyboard layouts you like whenever you like. :-)