Nah, Starbuck isn't the final Cylon. She's a hybrid, though, daughter of Tigh. Same personality type (hard drinking, fighting, etc), her mother was career military, it explains Adama's protectiveness (he knows that she's Tigh's daughter even if Tigh doesn't), and it explains her visions (because then she'd have shared code with one of the final five). No explanation for the shiny new Viper and her re-appearance yet, but I feel good about the rest of the theory.