Angled ring? Do you mean how the Halo looks flared in the title screen? I'm pretty sure that's just a camera distortion of some sort. As the camera flew around, the effect seemed to lessen and disappear. I strongly doubt they would have modeled it any way other than it has traditionally been modeled. I think that was a stylistic tweak of the viewing angle to make the shot look more interesting and convey a better sense of scale.

If that's not what you're talking about, I have no idea what is. The skyboxes look kosher to me. They even switched back while looking up and it more or less matched.