The scifi blog io9 has the teaser for the first two minutes of the Doctor Who Christmas Special on their site. I can't embed it here without doing some coding that isn't totally fair to io9, so just go there to watch it. Spoilers, of course!
It's pretty cool, and I think it bodes well for the special. The only complaint I have with it is how happy the Doctor seems to be as he steps out of the TARDIS. He wasn't exactly laughing with joy at the very end of season 4. He should have been somber at first then cheered up. Still, it looks like fun, which is what the specials are supposed to be.