Somhow I must have missed this one: Microsoft published SecondLight.
This is a system that is able to project a second image through the tabletop of a multi-touch screen onto a sheet of paper or whatever you want to use. But take a look at this video:
Actually, this is great. Anyway, I would not know what applications could use SecondLight except the obvious magic lens applications. Thus, I embrace following cite from the according SecondLight engagdet post:
Like the Surface, practical applications for this seem somewhat limited — but since when have you cared about practicality?