As Marge pointed out, it's been around for a while. I tried it a long time ago, but found it to be more gimmicky than actually useful. The faux-3D arrangement of windows on an essentially 2D surface didn't really feel right. I think that's one of the reasons why none of the mainstream operating systems have full 3D user interfaces yet; usability testing probably showed that it was more confusing than beneficial.
Now when there's an actual major paradigm-shift in the future, and we're no longer restricted to the current keyboard-mouse-display mode of operation, a true 3D UI might actually become truly usable. Think of a "UI" where you interact directly with virtual objects projected into a 3D space.