Okay...here's where I'm going to tie everything I've said in together, for my final "theory of everything" for BSG:
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The robotic Cylons, in keeping with the mythos of the show, were indeed created by the humans of the Twelve Colonies. As we've seen in "Razor," the original robotic Cylons were an artificial intelligence gone awry, and, after the first Cylon War, they retreated into obscurity.
The Final Five, being "fundamentally different" and in keeping with the mythos that they have "been to Earth," are an advanced form of human being (artificial intelligence?) that was created on the planet that they just landed on. The basestar Hybrids continually talk about everything having "happened before," so it is possible that the "Final Five" species were created here by humans, much as the mechanical Cylons were created by the Twelve Colonies. Equally cyclical, the Final Five species revolted and destroyed Earth in a global war, while a group of surviving humans packed up and left the planet in search for a new home, Kobol, and the Twelve Colonies. It is possible that the humans who left there and the humanoid "Final Five" species battled on Kobol, where they had their last stand, and, triumphantly, the victorious humans left for the Twelve Colonies.
Between Kobol and up to the time of the Twelve Colonies, the Final Five-types continually searched for them, looking for revenge or something of the sort, eventually discovering them shortly before the Cylon War. This is the timeframe where Saul Tigh comes into the picture, according to Adm. Adama. However, since there were so few of them and they were in no position to take on 20 billion people on their own, they instead discovered the robotic Cylons, co-opted them, and "evolved" them from the standard robotic Cylon Centurions that we see in "Razor" into the regenerative, highly-sentient Cylons we see now, in addition to using their knowledge to create the seven humanoid Cylon models as a fusion between them and the robotic Cylons, using 3,000 year old Earth DNA captured from their original war.
Four of the Final Five are, as expected, sleeper agents, but, even as they are increasingly aware of who they are, they are not fully activated. The fifth, as I believe now, is Ellen Tigh, but she was not present to become the Fifth by the end of last season, because she was killed. Or so we believe. It is my guess that Ellen, in keeping with a prophecy on the show...
"The fifth and final Cylon remains hidden, longing to be forgiven for some type of trespass, but will experience suffering in the process."
The "trespass" was her betrayal of Saul on New Caprica that got her killed. Couple this with the fact that, although she is dead, she appears repeatedly in visions on the show (how many deceased characters have been given this privilege? None, except Ellen.), and also, if you listen to the end of the following "hiatus teaser" that was aired on Sci-Fi after the mid-season finale...
YouTube - Battlestar Galactica Hiatus Teaser
...it sounds like Col. Tigh, in shock, saying, "You're the fifth?" Who could give him this level of shock? His dead wife, Ellen. Ellen, after being killed on New Caprica, was regenerated and has been hiding out back on the home planet, luring the BSG crew there to believe that it is Earth, and using some unknown means to manipulate people with dreams and visions, which I think will be answered in 2009.
Which leads me to my hypothesis that this is not Earth, which I base partly on what happened in the original "Battlestar Galactica" of the late 1970s. In that series, everyone had been lured to a planet that they believed to be Earth called "Terra" by "The Beings of Light," where their mission was to avert the planet's annihilation by nuclear war. They succeed, and then they are told that it is, indeed, not Earth, but that they are heading in the right direction.
In relation to the new series and the "Terra" hypothesis, the BSG crew has been heading towards "Earth" mainly on the basis of visions and other supernatural experiences, in addition to a 3,000 year old beacon of unknown origin, and a cryptic temple on the now-destroyed "Algae Planet." Considering how improbable it is for humans to have done either, I'm guessing that these were created and left by the "Final Five"-types to lure them to this planet "Terra" (or whatever), not to mention that all the visions and Kara Thrace's supernatural experiences were just as orchestrated to lure them to this planet that they believe to be Earth. This planet may be aptly described as the first planet that Earth humans colonized after leaving, and these Final Five-types were created for much of the same purposes as the mechanical Cylons were in relation to the Twelve Colonies, but, instead, were highly intelligent, which led them to the nuclear annihilation of everyone on the planet that they all just landed on.
I imagine that the last ten episodes of this season, starting in 2009, will involve the discovery of that Fifth Cylon on that planet, which will lead to the other Four being "awakened" and becoming completely evil. The hiatus teaser shows Saul pointing a gun at Adm. Adama, along with Adama weeping over a body. I believe that Saul, at this point, is awakened and attempts to kill Adama, where Adama, instead, kills him, which is where we see the weeping scene. However, I imagine that regeneration is possible on this planet (the Hub that they destroyed in space was only for the seven humanoid Cylons), and we'll see Saul again alive and well, but without that eye patch.
Eventually, with the secrets of this planet exposed and "the enemies" being vanquished once and for all, they will discover, amongst the ruins, the true location of Earth, and we'll get our "happy ending" in the series finale (which, I read, may be three hours long).
In terms of that bridge picture, I think it could just be everyone looking into it too much. If all the logic points to this being humankind having colonized farther into the universe thousands of years into the future, with their true origins having been lost to time and obscured through mythology, then what are the chances that the Brooklyn Bridge of today would even continue to exist, anymore than anything random from 3,000 years ago on our Earth would exist?
OKAY I'M DONE NOW! 2009 CAN'T COME SOON ENOUGH!