RDM and others have said they had the final cylon in mind writing of the first season. It has to be someone in Season 1.
Gaeta - its possible, but he has been used a red herring for the Cylon 7 and doubt they would use him. *spoilers* From what I have seen from season 4 previews the final four are going around doing various things, there is that one battle scene where they all are in the CSC, but if you see Gaeta he is just sitting doing his 'normal job'. I have also read some of the various info around the net and as RDM has said the audience wont even know who the 5th is until the 2nd half or later of the season.
The question is for any character what quality story can be had by the final cylon. Both Gaeta and Dualla are not interesting enough characters (in the grand scheme of the series) to be the Final cylon. I cant imagine what they have done that would make me go...oh thats cool/interesting with them as the final cylon.
Of the people who fit interesting, and in the first season that could be it I would say would be...
Bill Adama - fits, shocking, but probably too expected, plus i have a feeling he will be 'going down' with BSG sometime before the end
Lee Adama - fits, shocking, probably the 'effective' final cylon (if the final cylon is good), he fits with just about everything said (execpt for the 1 picture at the table which I think is meant to screw with us and to be taken with a grain of salt), he is the one person that is involved with EVERY main and almost every secondary character on the show.
Tom Zarrack - *spoilers* he is slated to come on in the 2nd half of the show which if you read above know thats when its supposed to be revealed, but thats also when the president dies, so he would also come back as the main president, so it works both ways* and I wouldnt put it past RDM to have had 'apollo' from the last series to be the final cylon, but i cant see them choosing him simply because of lack of screen time, sure he isnt that one picture, but I dont believe the final cylon isnt in that picture. if anything be guaranteed they ARE.
Baltar - fits with just about everything especially in first season, however plenty of cases why he wouldnt be (his quest to find out if he was), but he is visited by the 'angel of god' so anythings possible
Head Six/Head Baltar - remote possibility that the final cylon is the angel of god. if its true, from what i have read on the forums and else where i would be the only one to have guessed that, which may be RDM style.