Well, sort of. QuartzExtreme enables developers to easily integrate 3D and 2D content without a major CPU impact. So if you were implementing some sort of custom UI widget in your app, you'd be free to do it as a 3D object in OpenGL instead of drawing it using conventional 2D graphics APIs. Or instead of drawing flat bitmap icons in your UI, you could have 3D models instead. The documentation you're looking for is the stuff on "overlay/underlay surfaces" -- there's info on it in the NSOpenGL headers and a sample app or two in /Developer/Examples.
QE doesn't mean you can have 3D icons in the Finder/Dock or gain some sort of 3D user interface system-wide, if that's what you're asking. (Though it does open the possibility for Apple to provide such things in the future.)