One thing my hubby pointed out about the woman's appearance as we were discussing her possible identity (he had also originally assumed mother) was that although we have been culturally trained to assume a mother-son relationship due to their relative apparent ages, for Time Lords, that means precisely dick (to borrow from Men in Black).
My thoughts exactly. I keep thinking she might be the Doctor's wife. Either she somehow survived the revolt that lead the Doctor to leave Galifrey ages ago, or the Time War brought her back into existence (the line from the Councilwoman about living and dying and living again made me think that)
My brother pointed out something that I missed when I watched part 2, missed it the first and the second time I viewed the episode, when Wilf asked the Doctor the identity of the woman, the Doctor never gave an answer but he did look at Donna. It seems in the most subtle of ways RTD was hinting to us that that lady in white was like Donna's relationship to Wilf. Donna is Wilf's granddaughter so that would make the lady in white the Doctor's granddaughter, Susan. Add to this the Lady in White's line spoken to Wilf, "I was lost... so very long ago." This could easily be a reference to when The Doctor left her on earth in The Dalek Invasion of Earth. While I have no proof that this is the absolute answer I do believe that is the answer to her identity.