As far as I am aware, Magneto has been put on indefinite hold, replaced by the forthcoming X-Men First Class (with Michael Fassbender as Magneto) The trailer has just started doing the rounds on YoueTube:What I've heard (keep in mind that this is from back when the Wolverine movie came out) is that Xmen Origins: Magneto was supposed to be the next movie, but I haven't heard anything since the last one came out. Right now IMDB has nothing on it, which doesn't necessarily mean anything.
In other news, I've heard that a movie based on Deadpool is in production. If they don't butcher the storyline like they did in Wolverine's movie, this could be GREAT. Deadpool was always my favorite character in the X-men comics, with Gambit coming in a close 2nd.