The Pokemon Company released a gameplay trailer and updated their website with information about the upcoming free 3DS game, The Band of Thieves and 1000 Pokemon, that will tie in with the upcoming Diancie and the Cocoon of Destruction movie.
In the trailer below we see that the more Pokemon you collect via Street Pass, the better you will do in battle.
Those who download the game and play it at the movie theater will also unlock a special surprise for their Pokemon X or Pokemon Y game. This is probably how Diancie will be given out.
Check out the trailer and a gallery of screen shots below.