According to the tweet of Shikura Chiyomaru, the president of MAGES, Steins;Gate Movie
was announced to be premiered in the Spring of 2013. The title is "Steins;Gate Fuka Ryoiki no déjà-vu". Shikura said it will be a brand new original story about a case occurred after the true end of the game.
Steins;Gate movie official website
(new key visual)
Source: Shikura's Tweet
Story by symbv
Update Feb 1st
The premiere date was announced to be April 20th.
Source: Steins;Gate movie official website