I came across a mention of this today:
Cthulhu Shield for Arduino on Kickstarter.
It is like a low-resolution tongue-operated track-pad, useful mostly for gestures maybe. But the big thing is that it has feedback: electrodes can emit electrical pulses that would be sensed as touches by nerves in the tongue.
It could be very useful for people with paralysis in other parts of the body, but the designers have many other unusual uses in mind ...