Whonerdery
Okay, so I've seen a lot of speculation as to who the Lady in White was. General consensus seems to be that it was the Doctor's mother. 'k, fine, I can run with that.
Others have made significance of the fact that when Wilf asked at the end who she was, the Doctor looked at Donna's mother, and Donna, leading them to believe that it was Donna, as Doctor Donna, somehow looping back to enter Time Lord society at some point.
To which I say... why not both? It would neatly tie up the 'half-human' issue. It also meshes with the LiW's statement "I was once lost..." Doctor-Donna appeared, was lost, and then comes back later.
Others have said she was the White Guardian, or the emmisary of same. It would (with enough vague handwaving) explain how she could project out to Wilf when no one else could break the Time Lock. Again, why not that too? Donna regains her Doctor-hood, becomes an emissary of the WG, ends up looping back to enter Time Lord society and gives birth to the Doctor.
Which adds a certain poignancy to the interactions with Wilf "I'd be proud to be your/have you be my dad..." Well, one generation off, but...
Additionally... Donna is a formalism before a lady's name in Italian, and variants appear in other Latin-derived languages. And her last name is Noble. Lady Noble. Noble Lady. Could have been right there in front of us the entire time.