Why I vote for Gaeta as the final cylon (contains spoilers):
1) He was a pivotal member of the resistence on New Caprica. And look who represented the leaders of the resistence: the other four members of the final five! Coinkidink? I think not!
2) Everybody's looking at "which obvious cast member could it possibly be?!" If you look at a lesser character, the only ones who remain are Dualla, Hot Shot (both of whom have been pretty much AWOL so far) and Gaeta (whose only scenes have been pivotal to his character).
3) He's supposed to be the most human of the cylons, right? Sure Laura's got her dealing with Cancer bit going for her, but Gaeta just lost his leg! I think how he chose to stay conscious during its removal shows a lot about his humanity. He's also been so wounded emotionally by Baltar that he was willing to purger himself on the stand. I think that shows a decent portrayal of emotional suffering to go with the physical suffering.
4) Like the other four members of the final five, Gaeta was obsessed with a song at one point. For the other four, it was just prior to their being revealed as cylons. For Gaeta, it's just prior to the 3s revealing the final five.
5) Not a fifth reason. Just tossing in a random vote for Baltar's lawyer from the trial. Romo, IIRC?