IMO it's the Chief who's a Cylon. Him or Starbuck.
Yeah, I think Brother Cavell said he wasn't (being a Cylon himself), though he could have been lying. Though that would have made for an interesting hook-up between him and the first Sharon... both sleeper agent Cylons. We'll see.
The one person that I always hear mumblings about being a Cylon is Baltar. I really hope he is a human because of the fact that his character has to deal with the burden of indirectly causing the extermination of most of humanity. To make him a Cylon would diminish that fact. I think he embodies a lot of conflicts in his human nature: Whose side should he be on? He's out for himself and thinks only about himself.
Sam's an interesting choice. He was one of the best Pyramid players and humans would have looked up to him. I think Callie's out of it since she gave birth to a child with Tyrol (which also takes the chief out of the equation) and the Cylon's aren't too concerned about another human-Cylon child about. Any thoughts?