Director Peter Jackson has confirmed that there will now be three films based on The Hobbit, according to Deadline.  The third film will be released in the summer of 2014, meaning that there will not be a full year between the second and third film as there will be between the first and second.
Additional shooting will be required, and some material not from the original Hobbit novel by J.R.R. Tolkien, but rather from appendices published separately, will be used in the story (see "Three 'Hobbit' Movies?").
The first film, The Hobbit: The Unexpected Journey, opens December 14th (see "'The Hobbit' Movies Wrap").