At this point, I've pretty much given up trying to guess and just anticipate watching things unfold. The writers are starting to throw in suspects. In "Faith" Kara claims to hear music, Anders makes a deal out of Gaeta singing, and Baltar seduced Tory under constructing a metaphor based on music. However, the writers also led us to think the astronomical description Kara had seen was in the Solar system, so these could all be red herrings to distract us from the real suspect. Again - I've given up on the guessing game.
I can only offer my path of logic: The Significan Seven were all introduced as Cylons upon their first appearances, then four of the Final Five were revealed to be supporting characters. So I think the last Cylon has to be a very major
character. I'm going to be pissed if it turns out to be some one-shot character who we haven't heard crap from since Season 1. My only real suspects are Roslin and Zarek. Roslin because of the various curiosities surrounding her while Zarek has been very manipulative. Kara and Baltar have several factors in their favour, but from a creative stand-point, this would seem unfulfilling. I think Dualla is a weak suspect given how she has had no more than a whopping five minutes of screentime this season so far. The Adamas are the least likely suspects since the punch needed to deliver the surprise and all the consequences that goes along with it would be hampered by trying to explain why one is a Cylon, but the others aren't (remember: Cylons are not clones of real people).
By the way: I can't shake this suspicion that the last Cylon is fully aware of what they are and have been doing various things to propagate their agenda in secret. This doesn't really help clues since we really don't know what goals the Final Five have and where they intend to head things. It's part of this big chain of logic about the Final Five's ties to Earth, but there has to be a good reason as to why the last Cylon is being left in the dark - both from us as well as the other four secret Cylons.