The Final Cylon was not only not in the "Last Supper" picture (which Moore has stated more than once, now), but they were also not in the Colonial fleet (per 3's dialog) when the pilots, the President, and Baltar were being held hostage. Roslyn and Baltar are both in the picture, so that means its either one of the pilots, someone we last saw in the 12 Colonies in the mini, or someone we haven't seen yet.
Reading the reactions from some of the cast when they were told who the Final Cylon was, I am guessing its someone we would never guess. They all seemed to be expecting someone bigger, more prominent.
Anywho, just my 2 cents.