Here's an interview with Ian Mckellen from the website SuperHeroHype.com
He discusses the upcoming Magneto spin-off movie, and the new technology used in X-Men: The Last Stand, that will allow him to play himself at a much younger age.
I just saw X3, and was blown away by the effect used in the opening sequence of the film, where Magneto and Professor X appear in a flash back that is supposed to take place 20 years ago. It was absolutely subtle, and totally convincing. They really looked younger. Patrick Stewart looked just like he did in the first episodes of Star Trek: TNG. No makeup was used, instead photos of their younger selves were somehow mapped on to their faces.