I just had another look at this issue and finally solved it (for me anyway).
The problem seems to be the camera distance. If the camera is further away than 50,000 then the sorting doesn't work. The ring is displayed in front of the sphere if added to the scene second, or behind the sphere if added first. By reducing the camera distance to 50,000 or less the scene is rendered correctly - the ring going around the sphere. My project had the camera orbiting at 66,000 :-( Mark
