Got another possible clue along the same lines. In series 2 episode 8 (00:05.05) "From the darkness you must fall" is scrawled on the mirror in Tigh's quarters. The Final Cylon was decided on by about this point. It's a quote ("From the darkness you must fall. Failed and weak, to darkness all.") from a book D'Anna spots while reviewing news video rushes. Not sure if this shakes anything out.