In the shot of the Doctor when Wilf asks him who The Woman was, the Doctor's eyes are in three places. At the beginning of this specific shot, he is looking at Wilf as Wilf asks the question. Then his eyes move to the middle-ish, to right where Sylvia is standing next to Wilf. THEN his eyes move past Sylvia's position and focus outward into the distance, wherein we then see the shot of Donna, with Sylvia to the right out of focus.
Upon watching this very closely, I'm quite conclusive in my thoughts that when asked about the identity of the woman, the Doctor quite definitely looked at Sylvia THEN Donna, NOT straight from Wilf to Donna (as people have said in support of the "granddaughter/Susan" theory; I, too, was one who said it).
So.... yeah. I'll go with the "word of god" and say that it's the Doctor's mother.