It doesn't answer the surprise regneration question, but the BBC did release a special trailer for the final episode today.
It's centered around Davros (Dalek creator) and one of his psychotic rants. I will say that this new actor, Julian Bleach, portrays the mad scientist perfectly. He embodies the best aspects of Michael Wisher (first actor to play the role) and Terry Molloy (actor who has portrayed Davros on TV and in audio plays since the 80s).
