IMO the best application for this would be the ability to change from a regular to Dvorak keyboard just by flipping a switch. The Dvorak keyboard is the one someone invented around 1960 that is much easier to use than the regular one. (Aside: The keys were arranged the way they were on the standard keyboard to prevent the type bars for commonly used letters from jamming on each other.) I read a while ago that someone marketed a keyboard that converts from standard to Dvorak at the flip of a switch. But I think had to put a rubber thing over the keyboard to show the new letters on the keys. Using liquid crystal displays on the keys makes a lot more sense, though it would be expensive.

LCD displays would be costly, but in a couple years OLED technology should be very cheap and this would be possible.

Also, there are 'projected keyboards' coming to market:http://www.alpern.org/weblog/stories/2003/01/09/projectionKeyboards.html

If it is projected, then it is completely virtual. However, as someone who has used a traditional keyboard for nearly 30 years, I would be lost without the tactile feel of the key clicks.