I'm not exactly sure about this, but I seem to remember a declaration that "Cylons don't lie." Obviously, that's not entirely accurate, since there's been a lot of deceptions involving skinjobs since day one. But that's exactly the thing. Could it be that Cylons don't lie, they just deceive? Like Leoben, D'Anna, Caprica/Gina - they lie about themselves, but I don't recall them lying about stuff that are important to the show's mythology. So when Leoben said that "Adama is a Cylon," could it be that he wasn't lying? I'm not sure about Joseph Adama, but with Lee's startling turnaround towards the end of the third season, I'm starting to think he could be the Adama Leoben is talking about. Although that would create all sorts of problems since even D'Anna doesn't know who the final Cylon is, as well as it wouldn't (yet) explain Hera, Roslin and the Opera House.