Ian McKellen, Magneto in the X-Men films, had this to say recently about the planned third film: "I hear positive news about the second sequel, which is likely to include Magneto. The start date, considering Bryan Singer's other commitments, couldn't be much before September 2005. There is as yet no script nor am I signed up. On the other hand Hugh Jackman had a three-movie deal and Alan Cumming is also contracted to appear once more."
You can catch more of what McKellen wrote on his Epost.