RDM may not tell the whole truth (like Starbuck coming back), but that's not the same as outright lying. At least not to the fans. He knows we're trying to solve the mystery, and this is a game. Outright lying about who it is, or isn't, would be a cheat, and he wouldn't be forgiven for it. As a person who has been both fan and creator, he understands this relationship, and would be a fool to betray it.

Who have we been told is not a Cylon? Near as I can tell, just two rules apply:

1) RDM says this character was considered as far back as the first season, so it must be someone we know OR KNEW OF in Season One. This eliminates Romo, but allows in characters like Tom Zarek, Zack Adama, President Adar, etc.

2) It has been said directly that Ellen Tigh is not a Cylon.

We can also surmise that Cally and Helo are not Cylons as their children have been confirmed as hybrids (and we know who the cylon parent is).

RDM did say in a podcast that they ruled out Roslin and Adama as Cylons for the end of S3 as it would cheapen the story. Technically this applies only to the revelation of the 4 Dylans, but I don't see how revealing this in S4 would cheapen the story less (though it does at least allow them to continue with the cancer storyline). So while I wouldn't say these characters are ruled out completely, I think it makes it less likely.

For dramatic purposes, the character has to be one the audience can easily recognize - making it Racetrack or priestess who was killed in S2 would fail totally on the dramatic front. However, there aren't many characters that they can use to get a big surprise. Baltar and Starbuck are too obvious, and Apollo offers too many questions (like how did daddy Adama and mommy Adama have a baby Cylon?). Zack Adama offers the same problems as a Apollo, with the addition that we care a whole lot less than if it were Apollo. Also, given the whole Adama family history being established for Caprica, I think they don't want to go screwing around with it by making an Adama a cylon.

I really don't think they can do Baltar as a Cylon, as for dramatic purposes this would completely undermine the character's arc. Becoming a Cylon let's him off the hook for all he's done.

So I think this leaves Roslin (outside shot as I said earlier), Zarek, Gaeta, Cottle, Dualla and Billy. as the only characters that we would recognize immediately and thus would have real dramatic impact. However, of those I think the impact of Zarek and Cottle would be minimal, so I don't guess it will be them. Gaeta, Dualla and Billy all fit the "Dylan Four" profile of secondary characters who put themselves in positions to influence events and people in power. Of those three, I think Dualla is the most likely candidate as 1) we have been told there are plans for her this season, 2) Billy's dead and I don't think the audience much cares about him, and 3) the humanoid cylons have a severe female deficiency. (however, Roslin still remains my primary pick if RDM was misleading about her not being a cylon).

Let me toss out one more possibility, and that has to do with Starbuck. We know Starbuck is an obvious choice to be a Cylon, which is why many of us rule her out. But everything we've seen - from her skills to her "destiny" to her return from the dead- peg her as something special, and her mother, at least, knew this 20 years ago. What if Starbuck isn't a cylon, but a hybrid? WHat if she is the first hybrid? Then either her mother or her father would be the final Cylon and while we met neither one in S1, we certainly knew OF them in that we knew Kara had a mother and father, so they still fit RDM's rules). Revealing the final Cylon as an "unknown" character wouldn't have the dramatic visual, though Kara saying "Dad?" would obviously help that, and the drama would come not from their revelation but from learning that Kara's not all human (aside - we also have seen that the Cylons may have an attraction to other Cylons - witness Anders and Tory, Tyrol and Boomer - if Starbuck were a hybrid, you could add her and Anders and Leoban to the list). Now, of the two I would vote for Daddy being the Cylon - because 1) mom died 2)Kara's dad is a musician -like Dylan, 3)it could be his music that inspired her painting of the mandala, and 4) we all know girls love guys like Daddy - like Leoban and Anders. Now, if such a scenario - far fetched as it is - did turn out to be correct, I know how I'd love to see it play out. The final cylon is revealed in shadow - Kara calls out to him, identifies him as her father, and we cut around to see... Starbuck is Kara's dad. Good or bad, it would have fans pissing themselves. :p