I guess it's just natural that folks pondering the future of the GUI would turn to 3D spaces. After all, clearly 3D has played a dominant role in the most important software genre: videogames. However, past efforts towards the "3D Desktop" have seemed impractical. Nonetheless, I think BumpTop, which utilizes a tablet PC and pen tool, is on to something big. Watch the You Tube and see what you think.
I've been using a tablet PC for a year now, and have to admit that using the pen instead of a mouse has made certain tasks—particularly grading papers—a breeze.
As for bringing something like this to the desktop, I might offer Sandio's 3D Mouse as an appropriate tool. At any rate, these are innovations that might eventually present a "killer app" for GNU, provided they actually worked out well in practice.