The British Board of Film Classification have "cleared" a "Day of the Doctor" mini episode called "Night of the Doctor". The only details known are that it has been passed uncut, stars Matt Smith and David Tennant and is 6 minutes and 54 seconds long. It's classed as "additional production material" and is specifically called a "mini episode". Whether this will be a dvd/blu-ray release or will air online prior to the episode is unknown.
The BBFC also recently passed another piece of "additional material" called "The Last Day" - no cast is listed and it has a running time of 3 minutes and 57 seconds.