When Starbuck returned to Caprica to retrieve the arrow, she visited her old apartment and stumbled upon a recording of her father playing the piano. I only heard the song once, but to me, it sounds remarkably like the ballad that plays in the background during certain scenes on the cylon mothership. Hence, I like your theory of Starbuck being the first hybrid. This might also explain why Starbuck's mom was so abusive to her. Maybe she somehow knew that her daughter was a hybrid, and hated her for it. Take it one step further... what if humans and cylons somehow COLLABORATED to produce the first hybrid? That would be the mother of all conspiracy theories.