The Unknown Cylon

Okay, it probably isn't Dirk Benedict. :-)

My thoughts (before Revelations is aired, of course) are that it has to be

[drum roll]

Bill Adama.

Here is my line of reasoning:  in order to escape detection, the "final five" could not afford to be duplicated within the fleet.  Yet to survive, they all had to be associated with Galactica.  Only with Adama running the ship was there a reasonably good chance it would succeed.

Plus, it leaves a great unanswered question about Lee going into the mid-season finale.