According to the AICN review, the final cylon WILL be revealed in tonight's episode (though that scene was removed from the review copy of the episode - it comes at the end/toward the end).

I was betting on Gaeta, but after the webisodes and based on the "YouWillKnowTheTruth" website, it seems like they're trying real hard to make us think he is only to pull the rug out from under us. I think he's going to do something to sabotage the alliance between the rebel Cylons and Humans, but won't be a cylon himself.

I've changed my bet to Elosha. I know she's been dead since Season 2, but I think she's the original Cylon, living on Earth, controlling everything. Either that or it's Dualla. Guess we'll find out tonight.

--edit--

Wow. I was wrong.