We learn that the new episode is slated to be broadcast in the second half of Series 7. (Which means we could see this episode as early as Spring next year.).
However, Gaiman pointed out that The Doctor’s Wife, which he also wrote, was originally planned for series five and got bumped to series six for budgetary reasons.
He said that the same thing could easily happen to his new episode, so it sounds like this is another very expensive story to make.