Collected here are some bits of info on the Christmas Special and Series 6/7 from the In~Vision Commentaries on The Complete Fifth Series box set - read about the set HERE. The news comes from Steven Moffat on the The Eleventh Hour and Flesh & Stone commentaries.
"I've just written into the Christmas special a speech where he [The Doctor] talks about the fact that he doesn't have a hat."
"You might think, for instance, that there are things that live inside clouds. I'm just saying that, kids, as a little nod to the future there."
Moffat has also dismissed the "rumours" that Amy Pond and River Song are the same person and that River Song is The Doctor's mother.