Why

These are the key elements to the Zak Adama theory:

* Leoben said "Adama is a Cylon"
* Lee said something to his father about what would he do if he found out someone close to him was a Cylon

[exact quotes and episodes to come]

Zak as a Cylon would bring in compelling emotional complexity from Starbuck (since she was engaged to him and felt responsible for his death), Admiral Adama, and Lee Adama.
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It would also bring an element of surprise since everyone has been looking at a bunch of Usual Suspects on the ship:
* Baltar
* Starbuck
* Roslin
* etc.

On the element of surprise front, Admiral Cain as a Cylon would be cool too. Or Billy (the President's previous aide). Or Boxey. Well, ok, Billy or Boxey would actually be killer lame.

As has been pointed out elsewhere, Admiral Adama (or "Bill" to his friends) would seem unlikely to be a Cylon, since he survived fine on the station in the radiation that was killing Leoben. Other theories have Joseph Adama as a Cylon, which has timeline issues, or Bill's wife Carolanne - but then both Lee and Zak should be half-Cylon. Surely the final Cylon should also be a "full blooded" one?

I'm not even convinced all of the "four in hiding" that we have seen are Cylons.
Yes on Sam and the Chief, but I don't see how the timeline works for Tigh, and given recent events, it would be a lot more interesting if new President's Aide (whose name according to Battlestar Wiki is Tory Foster) turns out NOT to be a Cylon.

The issue then becomes that we have never seen a juvenile Cylon, and how did Zak become, well... born?

Possibilities:
* Zak switched in after birth by ? unknown Cylon overlord council ? (keep in mind Carolanne thought she was having a girl, both times)
* original human Zak replaced by Cylon at some point
* Zak as secret Cylon experiment, created in a tank but implanted into Carolanne

I don't read too many Galactica blog postings because I hate spoilers, but I really liked the detailed examination of the topic in

A “final Cylon” theory - TimeImmortal.net

I liked it so much that I mentioned it on Digg, and it got Diggdotted. You can still see it in Google Cache. (I should also perhaps attach a small disclaimer that the final cylon posting is just about the only one on TimeImmortal that I have any agreement with whatsoever.)