I can't believe we're not talking about this here!
If you haven't seen it, Spoilers, Sweetie!
When they showed all 12 doctors, oops--"No sir, all 13" I jumped up and screamed "Yes!"
At the end when Tom Baker appeared, I did some more jumping around. That was brilliant. I'm amazed it was kept under wraps.
My theory is that he represents the Doctor when he retires, following up on when #11 (or is he #12 now) saying "I could be a curator." When he knows he can regenerate no more, he chooses his favorite body to live out the remainder of his life.
The only disappointment is that the regeneration of the War Doctor did not result in an appearance of Christopher Eccleston, #9. That would have made the show perfect.
Speaking of, I had an almost inappropriate dream about Chris. I was at some sort of Doctor Who conference, and I was walking with him to an event. I told him that he was my favorite Doctor, and that I was disappointed that he didn't have more than one season. Apparently that put a spark in him, because as we entered the audience and sat down, quieting ourselves for whatever presentation to begin, he turned to me and said, suggestively, "We're not done."
Not that I find him attractive at all. It's true that he's my favorite from the reboot. But frankly, I don't think any of the new ones are that good looking. I know David Tennant is supposed to be some sort of heartthrob, but he's just way too skinny and his nose is weird. And Chris and Matt are just funny looking.
Peter Capaldi, on the other hand....I'm ready for this mature doctor! In fact, that goes along with another one of my (completely uninformed) theories. I haven't seen any of series 7, but I know that there is a mystery about who Clara is. I say it circles back to the first Doctor, and she's his granddaughter. That's why they casted someone older. I'm probably wrong, but we'll see!