Now that many pre-showings of Movie 11 have concluded in Japan, many people have already watched the latest Movie, Giratina and the Sky Bouquet: Shaymin. At the end of each movie they air a preview for the next years movie. According a Japanese blog user who attended one of the pre-showing, Movie 12 will be called 'Theaterical Version Pocket Monster Diamond Pearl Platinum'. This title also brings up another interesting bit of information. The titles of the movies generally reflect the title of the current anime series, which means its possible that the current 'Diamond & Pearl' series in Japan will be replaced with 'Diamond Pearl Platinum'. It is also assumed that Arceus will also be in the movie title but that won't be confirmed until around March 2009.

We will be able to confirm this information later this week, but there is no reason to discredit this information as many people in Japan have already seen the movie and would have reported the information as inaccurate.

Source: http://ameblo.jp/mineie-hikozaru/entry-10113487206.html