This is hard to describe, so I've attached a screenshot.
For some reason, the Horus never seems to get quite as dark as anything around it. This screenshot illustrates the effect pretty clearly. This happens using a 3.6.10 trunk build and the 3.6.10 mediavps. This particular screenshot is from the Transcend campaign (using mediavps), but I know I've seen it elsewhere (Procyon Insurgency at the very least).
I think I've seen something similar with certain faces/angles on the GTI Arcadia, but I'm not as sure about that and I don't have a handy screenshot.

[attachment deleted by ninja]