Hate to be a party pooper but unless im mistaken this doesnt work in a true 3d scene with a moving camera. It looks fine with not much else in a scene and orbiting the camera but if you set up static reference points and animate a camera panning around the outside of the scene you will see that the sphere doesnt act correctly.
I believe this is due to the lookat expression producing 0-360 degree orientation values but not revs and degrees required of a rotation value.
Try it and see for yourself.
Next question is: whats the expression for a truly 3d sphere which can be panned around correctly?