It was expected he would depart sometime in the next year or so, this soon however is a bit of a surprise.
The BBC have officially announced that Matt Smith will depart the role of The Doctor on "Doctor Who" after three full seasons and four years on the hit show. Smith's final appearances will be this year's Anniversary and Christmas specials in November and December.
Smith has been an interesting choice for the eleventh incarnation of the character, a more quirky and character driven performance than his immediate two predecessors which has divided long-term fans.
On the flip side his incarnation has brought in various new fans to the fold, especially outside the U.K. (and more specifically in the United States) where the show's fan base has never been more mainstream.
Smith's inaugral fifth season of the show since the reboot was widely acclaimed. The subsequent heavily serialised sixth season and all stand alone adventures of the seventh season have received a more mixed response.
In his time though the actor has been a part of some highly regarded episodes such as "The Doctor's Wife," "The Eleventh Hour," "Vincent and the Doctor," "The Pandorica Opens/The Big Bang," "The Impossible Astronaut/Day of the Moon," "A Good Man Goes To War" and "The Name of the Doctor".
The search now begins for the twelfth incarnation of the character whom will likely be revealed before the Anniversary Special. Steven Moffat is expected to remain as show runner.
Source: The BBC