(and no, this isn’t spoiler-blocked — the show has aired in the UK, and it’s aired in the US. Past tense.)
It’s been confirmed in the audio commentary to the End of Time Part 2 that Russell Davies never intended to reveal who the “Woman in White” is — she’s apparently whomever you want her to be.
Russell Davies can Choke on My Fuck.
Comparisons are being made to classic McGuffins like the briefcase in Pulp Fiction. If so, then Rusty needs to be taught what one is — because they’re supposed to be items which are meaningless outside of their role as an incidental plot focus. The classic example given is secret papers in a spy story — what’s on the paper is meaningless; the fact that the characters all want the papers is all that matters.
McGuffins are not characters who perform critical actions to the plot, as the “Woman in White” does. She appears to Wilf (outside of the Time Lock, so she’s obviously more than just a Time Lord, despite appearing with them at the end), prompts him to go searching for the Doctor. She appears again, getting Wilf to get the gun which he will give to the Doctor, critical to the plot resolution. She appears to the Doctor at the end, and it’s his recognition of her which leads him to make the decision to fire, as well as the decision of his target.
But she’s “whomever you want her to be.”
BULLSHIT. That’s nothing more than lazy writing.
So, I guess the true identity of the “Woman in White” is actually RTD’s deus-ex-machina bullshit, personified.