First of all, let me say this is the worst possible way to spoil anything up. Second, whinning about Smith for a frickin' cameo
just seems too much to me. Calm the fuck down.
OK, with that out of the way... I kinda like this. I like that the Doctor, before his regeneration, wanted to make her feel at ease with his future self. It reminds me of Nine trying to ease Rose in the change to Ten.
That said.. I'm not sure its gonna be a good idea to have a past Doctor basically going "don't worry kids, I may look like a doddery old grandpa, but I'm still me!". On the one hand, I understand Moffat's doing it for the younger audience to stay tuned for this new Doctor, but it does seem out of the ordinary. I mean, we didn't have Jon Pertwee or David Tennant reaffirming their audiences to stay tuned for this new Doctor, did we?
Anyway, while I like it as an idea, I guess we'll have to wait and see how it'll play out.