Worldspace Normals for Material Nodes?

Is there a way to get the worldspace normals as the input for material nodes? I was looking trough the possible inputs, but all deliver normals in screenspace (relative to viewer) or just the coordinates, but not the normals in worldspace.

In my case i wanted to change the shading depending on the fact if a face is directed at the sky (positive z-axis) or not. But it does not seam to be possible, since what is up or down is defined by the camera inside screenspace.

Some way to get the right normals im seeking?