TV Further to yesterday's overcompensation some other ideas about who Jenna-Louise Colman's playing:
(1) What if she's like Scaroth from The City of Death? Aspects of her are scattered across time and rather than travelling in the TARDIS she turns up anew in each episode ala the one off companions in most of the specials. Perhaps the Doctor finds out where she is somehow or how to track her and makes it his mission to unify the pieces ala The Key To Time (because she keeps saving his life). Each time he lands she doesn't know who he is yet.
(2) Let's not forget that silence will still fall when the question is asked. Perhaps she's the one who's going to ask it. Although to be fair the Daleks asked it a lot in this episode. Perhaps it's to do with the tone of voice.
(3) It really is just that she was cast for just this episode and will turn up later as a completely unrelated character like Eve Myles on a tiny time-scale.