So here's mine.
They said that "All this has happened before, and will happen again". What could that mean?
Answer: Humans on Earth created Cylons, who took over Earth. Humans at the 12 colonies also made Cylons, and they too turned on the humans. Don't be surprised if it ends with them on a new planet, wiith the cycle somehow continueing on.
Ellen is a 6 because:
1. Tigh was all over her, seeing her change back and forth between Ellen and 6.
2. Ellen was manipulative to Tigh, just like 6 was to Baltar.
3. They gave away that Ellen was a cylon way too easily, no way is she THE cylon.
Dee is the final Cylon because:
1. She was upset while seeing the jacks and watch on the beach, because she too was having a flashback (they just didn't show it).
2. She looked at the picture of herself as a child, and said something like she didn't remember that, and that they never expected to be who they are today.
3. She killed herself because of what she found out she was.
EDIT: Ok, so this intervue would appear to blow these theories out of the water: http://featuresblogs.chicagotribune.com/entertainment_tv/2009/01/final-fifth-cylon-ellen-tigh-battlestar-galactica-dualla-dee-.html
But it's either a really well-played trick, or just a really lame choice for and reveal of the final Cylon...
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