Bleeding Cool has reported that they have it on ''very good authority'' that Paul McGann, the Eight Doctor, has filmed something new for the shows 50th anniversary. This news comes after the rumours from the Radio Times which suggested that an older Doctor could return to the show for the 50th.
It states that the rumours are of two, either he has filmed something new for a section of the 50th episode when all Doctors will feature. Or that he has filmed ''a minisode prologue'' for the 50th episode, perhaps a regeneration into John Hurt. It also states that it could be both of these things or neither. As always take this with a pinch of salt.
More details here.