Reading between the lines of the mid-season finale of Battlestar Galactica, we can narrow down the possibilities for the Final Cylon a little bit further. Taking Xena at her word that there are actually only four Final Cylons in the fleet, this suggests that the remaining Cylon was either dead or already on the basestar.
Already on the basestar:I seriously doubt it's Adama, and it really shouldn't be Baltar, for reasons I've already talked about. Helo has always been someone everyone assumed couldn't possibly be the final Cylon because of the Cylons-can't-have-babies issue—which makes him a pretty interesting choice, especially since that assumption's now out the window with the Tigh-Six pregnancy.
Roslin, Adama, Baltar, Helo
Cally, Billy, Kendra Shaw & Admiral Cain, a host of others
Most of the clues right now seem to be pointing as Roslin, but I still like Cally. As far as I can tell she's the character whose return/revelation as the Final Cylon would open the most dramatic doors, and in particular would really point the way towards a realization of the human/Cylon reconciliation plot that appears to be the main thematic for the season.
Good episode tonight, by the way. I was spoiled on the Earth revelation from way back, but I was still moved by the sheer nihilism of the ending shot. Even if it seems a little strange that these characters and only these characters would be the landing party, the final long-take was beautifully, beautifully composed. And while we didn't see a tell-tale Statue of Liberty head, I'm definitely with those who think those were the ruins of the Brooklyn Bridge.