I dont know if this has been discussed but I was thinking about how something like this could be possible, and thought about transparent LCD displays.
Now I believe but may be wrong that LCDs are transparent untill current is passed through the screen making them opaque.
(a source for this)
So if Nintendo had 3 or 4 layered LCD screens that can have images on them while leaving areas transparent this could create a real illusion of depth.
It would also have the benefit of not requiring to be constantly moved to appear 3D. Cost may be a factor however.
Obviously an object would be on one layer however and would not have any depth itself, but this could also be simulated with the motion sensor in the device. I could imagin this working really well in a Monkey ball style game, tilting the platform from above.
Thoughts?