A few things to consider. I believe that any character with a verifiable history cannot be a Cylon. Especially those whose pasts have been intermingled. Recall, Boomer's whole family supposedly died in some mining accident. During one episode, Baltar drugs Galen and forces Boomer to disclose that there are 8 Cylons were / are deployed in the fleet (at the time before settling on New Caprica). Seeing how we know of 7 models already, I'm thinking only 1 Cylon model is left deployed in the fleet. It has not been established what the significance of the final 5 Cylon humanoid models is. All we know is that they *are* significant - which leads me to believe the final Cylon in the fleet will be a "biggie" and possibly not a secondary character or a "sleeper" agent. Also, the present 7 Cylon models do not know what the other 5 look like (at least D'Anna does not), which means on New Caprica it is possible the final 1 Cylon model deployed in the fleet might have stay deployed. Finally, we know the present 7 Cylon humanoid models by and large cannot procreate, so it may be safe to assume the final 5 cannot either. In Athena's case, it sounds like she is the first and that her offspring is very unique / significant as a result.
Here is my break-down as to who can be a Cylon:
William Adama - NO, too much history in military, had sons, would not make sense plot-wise, Baltar Cylon test would have revealed (first candidate)
Laura Roslin - NO, too much history in politics, would not make sense plot-wise, Baltar Cylon test would have revealed (first candidate)
Gauis Baltar - MAYBE, SEE BELOW
Lee Adama - NO, too much history with family, has father
Kara Thrace - NO, too much history with Adama family
Karl Agathon - NO, if yes would be no Cylon / Human hybrid
Saul Tigh - NO, leader of the resistance on New Caprica, tortured pretty badly by Cylons, too much history in military
Galen Tyrol - NO, established in one episode with Brother Cavil
Felix Gaeta - MAYBE, no history, but too obvious, probably red herring
Anastasia Dualla - NO, to quote the actress in an interview: "That would mean every hot girl on BSG is a Cylon"
Tom Zarek - NO, too much history as dissident
Ellen - NO, too much history with Saul unless she is a replica or a real person, too obvious, red herring
Billy - NO, contract disputes with employers usually means you are never coming back (and he is no David Duchoveny)
Kat - NO, too much history as recently established
Anders - NO, leader of resistance on Caprica, too much history as sports icon
Cally - NO, too pregnant
Admiral Cain - NO, given the immense harm she has done to the Cylons, too much history in military - but this one would absolutely blow my mind
Sebastian Spence - MAYBE, I agree with the comment about anyone having their own SCI-FI show
On a side-note, I think it would be a shame / waste if Sebastian Spence was not given a significant role in BSG. He has proven acting ability, charisma and star-power.
You may notice, I chickened out from saying a definite YES because I do not think we have enough info at the moment. However, if I were to venture a guess I'd say Baltar is a Cylon. However, there is too much contradictory evidence thus far. Baltar has too much history as famous scientist. Baltar's relationship / manipulation by Caprica Six on Caprica triggering the destruction of mankind would be trivialized / nonsensical if he were a Cylon. Baltar is way too eccentric to be a Cylon - he is extremely emotive and seems to represents base human traits - positive and negative. Still, Baltar walked out of his cottage without a scratch after the nuclear blast. Baltar seems to be able to think or "project" like the Cylons - with Six-In-The-Head being a possible manifestation of that. Baltar seems to have earned a special place amongst the Cylons (currently its lapdog / sexy pool boy but he may move up the hierarchy) - he has not been killed yet and I don't think any other Humans were able to tag along from New Caprica. Finally, the Hybrid recognizes Baltar as being some one of significance or "intelligence".
To end, my posts are too long.
