With the success of the anime series, which just ended yesterday at episode 24, a teaser of the upcoming film was revealed at the end of the episode:
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5pb. studio head/producer Chiyomaru Shikura had revealed the news before the announcement actually aired. Steins;Gate producer Yoshinao Doi had also indicated earlier on Tuesday that viewers should keep watching their televisions after the episode's ending animation sequence is over.
There's still no date of release yet, but it can be highly likely for 2012!