An official synopsis of the movie Magneto, aka X-Men Origins: Magneto, has shown up via the Coventry Telegraph (here):
The original X-Men Origins film began with a prologue that showed the character as a child being led to a concentration camp by Nazis and that is the period in which the Magneto film will take place. This setup will allow a future villain to at least flirt with the designation of protagonist since the character will be seen almost exclusively in his formative years.
The storyline will heavily involve Professor X, the wheelchair-using X-Men leader. That character was a soldier in the allied force that liberated the concentration camps. The professor meets Magneto after the war and while they bond over the realization that they are alike in their special powers, their differences soon turn them into enemies.
Magneto in his ultimate quest for power will come to share the same kind of eugenic ideals that once brought the young Jewish boy he was to a Nazi death camp...
But this movie will probably focus longer on its formative years. When the anti-hero was not yet completely twisted by the hardship of his life.