This is the second article I've seen this week on double pane display technologies. No, I am not talking about one pane beside the other, but one display on top of the other.
This article shows various techniques achieved, as well as display types. It seems to me the same effect could be achieved using other methods, but it is still interesting to think about options.
The other article I saw was from the Microsoft labs on a new version of their Surface display (which I've written about before). They project a second display above the plane of the Surface device. The technique is called SecondLight and needs to be seen to be understood.
I am not sure what it will be used for, but it does make me wonder.
I actually was able to spend quite a bit of time playing with the Surface device a few weeks back at the PDC. There were a number of them scattered around and they had a scavenger hunt game where you had to visit many of them and have your information collected along the way, since the Surface device can read information from items placed upon it. You were given a card that acted as a key to log your activities.