Expose folders would be as 3d as I could handle
Whilst we have 2d screens as our window onto the computer world, the only appeal I can see of the 3D idea is having a 3D cube as the format of an expose style folder. Say you have a project, "My Presentation", you could have a combination of up to six apps, browser and draft with their settings saved exactly as you left them the last time you worked on the project. One click and the cube turns into six thumbnails like now in Expose and you are ready to rock.
But a folder is the limits of a 3d cube IMHO because in 2d media land, no techy has ever out invented the intuitive nature or the simplicity of viewing a book held in the hand Same applies to screens for viewing slides and movies. Even if you enjoy reading newspapers over peoples shoulders or watching high speed chases beamed onto 360 degree screens it is disorienting, which isnt good for productivity.
Expose's set of 6 2 d thumbnails would be far more effective for viewing and dragging and dropping than working with off skew pictures on a revolving cube .
Expose is not some futuristic dream.