You know the thought just occurred to me that the Final Cylon probably isn't from the Earth we saw in the last episode. Rather it's from "Earth 2." That would explain why the Four heard the music but the Fifth didn't. It would also explain the comments that the reveal will set the direction for the rest of the season. It's the key to finding "Earth2."
I kind of always thought that the "final five" were some sort of reincarnating cylons who could pass their consciousness down from one generation to the next. This would explain how they predate the original seven, but are still possessed with a history young enough to have "always" been human.
At this point, I think that logically the Final Five all but HAVE to be "memetic" Cylon entities that incarnate in human forms each generation, or when they choose to incarnate. Their physical Cylon abilities could be explained as the human genetic code being modified to optimize the body for Cylon purposes.
Any other explanation I can imagine would require tortured retcon and stretching plausibility into the Deep Crazy.
However, even if they were born naturally in their current forms, in the colonies, their origin could still be Earth or Earth2 - just in their original lives.
I have a further theory though.
On one hand, I dislike the idea of the remaining core human characters being the Final Cylon - Roslin, Adama, Starbuck, Apollo, Baltar. This would run the risk of undercutting the human side of the drama. But... if I had to pick one, I'd pick Starbuck for the following reasons:
1. Starbuck reincarnated in a way that is hinted not to be like a skinjob Cylon.
2. Starbuck had an immediate "connection" with Earth when she saw it. "It was like I never left the place."
3. Only four of the Final Five were known to be in the fleet, unless D'anna was lying to throw people off.
Now, consider carefully: Starbuck was worried about losing her intuitive lock on Earth if the fleet went too far away. While out searching for Earth, she was also stressed at not being able to regain her lock. But she didn't find Earth by regaining her own sense of direction; she found it thanks to running across Leoben, and then out of desperation, lucking across the distress call in her Viper. Her Viper led the fleet to Earth. Not her original, let's assume true, feeling about where Earth was.
Result? The nuked Earth the fleet has found is NOT the world Starbuck was shown during her death and ressurection. She found the real Earth, or Earth2, or final Earth - whatever. She has not yet regained her instinctive lock, and for the momet is too shocked by what is happening to realize that the nuked world she is left standing on does not feel right. Starbuck under this scenario is the Final Cylon, and she's from the true Earth, unlike the Four.
For the moment, I imagine something like:
The Five, as a whole, are a group of Cylon minds that stepped in to safeguard humanity in the colonies, or at least to observe and watch them, not necessarily for sinister purposes. But it was a joint effort between the Cylons of Revelations Earth, and a representative of /Earth/ - Kara Thrace. Perhaps the location of /Earth/-Earth had been lost and Starbuck was a Cylon survivor from it, thus was on Revelations Earth when whatever happened, happened.
Whatever it is, for the moment I am almost certain that the crucial plot point is that Starbuck didn't rely on her instincts to find Earth and the Viper signal has fooled everyone including her, and that her empathic connection to Earth is because she's the Final, and is indeed originally from Earth.
That, and I suspect one of the twists with the nature of the Cylons is that the Final Five, or Mind Cylons, or what have you, may end up being nothing more than developed, evolved humanity having come up with a method to transfer consciousness, etc. So in the end, the Five may be more human than anyone we've seen in the entire show; the real nature of the word "Cylon" might play into this. What the Colonials think of as Cylon is something of a warped and twisted thing.