Although the Christmas Special doesn't start filming until next month, SFX magazine have done an interview with Steven Moffat (published 24th August). Steven says this about the Christmas special.
“It will be as Christmassy as Hell. I might be cleverly redeploying a known Christmas story again, sort of… but probably not the one you think! It will be massively Christmassy and moving and sentimental and sweet. It’ll be set partly on Earth, but not completely. And it might have a period setting…”