Many props to Chris for keeping us in the loop on the BBC calendar. I'm going to jump in here and add one additional link. The fan blog Whovian.net has also launched it's own adventure calendar, and I think it's worth mentioning here as well because they've also done stuff like create their own Christmas cards and they have also started uploading a serialized short story as well. This shouldn't be confused with the official BBC-related material, so I won't plug the thread with links. The Whovian advent calender can be found on the site's homepage at http://www.whoviannet.co.uk
. Day 3 is of particular note as it contains Part 1 of the short story "A Leadworth Christmas". So far, all the material has been accessible internationally; since they are not connected to the BBC, I would assume this won't be an issue with whatever they end up posting.
They seem to be following the same type of schedule as the BBC, though, in that they don't actually update the calendar until later in the day.
I'm also keeping an eye on Paul Cornell's blog (paulcornell.com) as last year he did a 12 Blogs of Christmas in which he snuck in an exclusive short story called The Last Doctor.