According to the Tweet of MAGES’s president Shikura Chiyomaru, the movie of STEINS;GATE will be premiered in Spring 2013. With a subtitle of Fuka Ryoiki no déjà-vu (負荷領域のデジャヴ), the movie was firstly announced last year at the end of the last episode of the TV Anime. The movie was scheduled to be premiered in this Fall but it turned out to be a printing mistake. The president also pointed out that the story will be an original one. The official website of the movie was launched and you can check it out for more information.
October 10, 2012 Sorry for the error. It should be in Spring 2013 instead of January 2013 in the title.
Meanwhile, the TV Anime of Robotics;Notes, the third game of 5pb. after STEINS;GATE, will be premiered in this week and you are waiting impatiently for next Spring, you might want to check this out and see if it fits you well as STEINS;GATE.