I really can't see them going that long without springing a leak, but even if they do, the surprise will only work for the British audience, which granted it's a British show but it does have a worldwide audience. The east coast of North America, the next ones who'll be watching the special after it airs in Britain are what, three or four hours behind Britain? By the time the special airs in Britain, it'll be about five or six pm eastern North America, and we'll have to wait until nine or ten our time to watch it, meaning we'll be able to find out the new Doctor online nearly five hours before we even watch the special.Otherwise, I think there's a real chance that they're going to attempt to keep the identity of the new Doctor secret until the Christmas special airs.