While IMDB is not always accurate, there have also been reports that a screening of the Man Of Steel was recently held for the higher level Warner Bros. executives. It would make sense that the running time information could show up on IMDB after a screening of the film.
Given that the Man Of Steel is both a re-telling of Superman's origin story and deals with an epic battle with Zod, the Man Of Steel would likely need to be about two and a half hours in length to fit everything in. The 148 minute running time would also put Man of Steel just above the running time of the original 1978 Superman movie, which had a running time of 143 minutes. It would also make the Man Of Steel slightly shorter than the 2006 Superman Returns, which had a running time of 154 minutes.
Man of Steel is set to be released in movie theaters in the United States on June 14, 2013.