Okay, I've seen a few negative reviews of the half-season finale (I dislike calling it a finale since it's just the halfway mark of the season) but I thought the episode was outstanding. And that ending, that jaw-dropping ending, I so did not see that coming. I knew they were not just going to find Earth and end on a happy note, but damn. I have to say Battlestar Galactica is possibly my favorite sci-fi show all time.
Anyway, we still do not know who the last of the final five Cylons is. Or where the last one is either. The D'Anna said the last one was not in the fleet. So where is he/she/it? Rumors are rampant. Mostly centering on people who have died in the show. Some folks speculate it might be the long dead Adama son Zack. I'm guess it is someone we've seen before, but maybe not for a while. Who that would be, I don't know. I expect it to be a shocker, but I also trust Ron Moore and pals not to pull a deus ex machina on us.
To address Sirs' question, I think the idea is that the final five are significantly different from the other seven. They were hidden with a reason. I feel safe in saying that because the other seven models were not supposed to think about them, according to the model played by Dean Stockwell. So I think if we could answer the question of why they were hidden, we might have a clue as to who the last of the final five is. The repeated line "all of this has happened before and will happen again" speaks to a hidden entity manipulating results. Running a program or experiment repeatedly to see what the results will be. Exactly what that means for our band of humanity trying to survive, I have no idea.