To deviate a bit from discussion, I've been theorizing about Saul Tigh's status as a Cylon, and how it possibly disturbs (or completely destroys) our current understanding of Cylon origin.
I think Tigh's age should be questioned first, as it came to my mind first as soon as he was revealed as a Cylon in the Season 3 finale. Through direct character development from other characters on the show (Adama, some flashbacks), Tigh is said to have started his military career as a lowly deckhand; however, he climbed the military rank ladder all the way up to Chief Petty Officer by the beginning of the First Cylon War. (http://en.battlestarwiki.org/wiki/Saul_Tigh#_note-deckhand). This, of course, brings up a striking question; how could a humanoid Cylon have existed before the first Cylon war? Tigh's very existence inherently throws away our previous understandings of Cylon evolution (the previously understood story being the original Cylon models started to "experiment" on bodies captured during the Cylon War, eventually creating the first "hybrid", which let to the humanoid Cylon models).
Secondly, and arguably less possible/important, is Tigh's susceptibility to perceived Cylon weaknesses (or lack thereof). For example, Tigh's extensive military career, which no doubt required him to traverse the planetary system of the colonies frequently, would have (and should have) exposed him to the high radiation levels observed in certain places in the planetary system (such as Ragnar, which made the Leoben Cylon seen during the miniseries die). By our current understanding of Cylon physiology, Tigh should also be highly susceptible to other viruses or bacteria that humans have developed an immunity to (i.e. the sickness which killed all of the humanoid Cylons on the Basestar which picked up the beacon on-board). Improbably possibilities aside, how could Tigh have avoided radiation sickness, and other diseases?
All of this, by nature, invites a slew of possible theories about true Cylon nature, and having proposed these question, I feel it necessary to put forth one of my own theories .
To start off, I believe that Tigh being a humanoid Cylon completely offsets the previously understood notion that Cylons were entirely created by humans from the 12 Colonies, and that humanoid Cylons were entirely created by the aforementioned colonial-created Cylons. That being said, I think it's logical to say that the Final Five Cylons (or at least Saul) came from another place altogether; Earth. Perhaps the time-line that the characters perceive on the show, fueled by religious scriptures, is in fact wrong, or backwards; perhaps the original humans came from Earth, and traveled outward from there, eventually reaching Kobol.
Another proponent to this theory is the phrase, frequently repeated: "All this has happened before, and will happen again". Perhaps the original humans from Earth created Cylons, which eventually rebelled, forcing them to look for a new home (Kobol). Original Cylons from Earth (reminiscent of the "Final Five") could have then pursued the humans to Kobol, underhandedly spearheading the advancement of Cylon technology, and repeating the devastating cycle once more, leading to the formation of the colonies. The cycle, in its third iteration, now finds humans pursuing Earth, which may end culminate in the humans settling in another planet altogether, rebuilding society, only to eventually repeat the destructive cycle again, put on by Cylon infiltrators from Earth.
These questions, and this theory (although incomplete ) definitely requires us to look at Cylon origin anew. What exactly does it mean to be a "Cylon"? Were the Final Five Cylons created in the same fashion as the other humanoid Cylons? If not, where did the other Five Cylons come from? All thought-provoking questions, and I myself will be watching Season 4 of BSG intently, waiting for them to be answered.