I remember there was a discussion about that going on at dean_sam once and people seemed to agree that his eyes aren't fully black in that scene.

Well, I stay away from dean_sam because of all the spoilers. But they're right...and wrong.

Jared wasn't wearing the black contacts they use for demons, like he did in BUABS. Those are contact lenses; if you look real close you can see the whites of the eyes at the very edges.

What they did with the Samhain scene was CGI. It's subtle, which is why I agree it might be meant as a tease and Sam's eyes didn't really change. But if you study it real close in hi-def you can see that it's not just the lighting. In one shot there is a visible change.