I watched the marathon of episodes from Seasons 1, 2, & 3 and I have to wonder if they were thinking of Tyrol as Cylon from the beginning. He makes love to the vipers the way Boomer made love to the captured Raider in Season 1.
As for the 12th Cylon, I don't think we've seen it yet, unless it is Romo Lamkin (Baltar's second lawyer that seduced Lee away from his father's side). The picture that's up on Scifil.com right now has been referenced by Ron Moore as NOT containing the 12th model, which leaves Lee, Kara, Bill, and Laura all out of the mix. Carl (helo) too for that matter.
Oh, and the first episode, there was a scene where Torrie, Laura, Saul, Galen, Bill, and Lee were talking about Starbuck's ship and throwing out theories. Notice how the Cylons in that group kept on discounting any theory that would point to Starbuck having really gone to Earth.