Of course, as I've said elsewhere, Matt Smith said to Jonathan Ross, when asked to describe this story in one word, 'pictures.' That could mean we'll see pictures of the older Doctors. But I can't help wondering if there'll be some sort of Dorian Gray aspect to the story. Portraits, probably holographic, of the earlier Doctors in the TARDIS, which age, even as the incumbent remains young and vital. Could this be a way of having Tom, Peter and Colin back on board?
Ooh I like that idea...
Even if Ten/Eleven is all we get from an actual Doctor perspective, I'm still hoping we might get a classic companion. There'd certainly be no issues over them looking too old because most of them didn't die, they just left the Tardis, so it doesn't matter that Ace/Leela/Tegan/Turlough or whoever look older/fatter/thinner/greyer, whatever.
I think it's a given that, were she still with us, Elisabeth Sladen would have been in this