Nothing has been made official yet, but it looks like the next movie in the X-Men Origin series will be Magneto. It is slated for a 2011 release.
It will follow the Star Wars movies where they highlight a characters path to evil. The Magneto character has a magnificent back story. He is a Jewish Holocaust survivor whose actions are driven by the purpose of protecting the mutant race from suffering a similar fate. You might remember some of his back story which was teased during the opening sequence of the first X-Men movie. It is going to be great to see the whole thing unfold.
IMDB Description: A look at the early life of Eric Lensherr and his friendship with Charles Xavier, and how the one-time allies became mortal enemies.