From comicbookresources.com today:
At the Avatar premiere, director Bryan Singer told various people he has signed to direct the film. He just hasn't told any of the usual suspects. Collider points to Flickchart's Twitter page, but the post is without quote. IGN points to a video at MySpace where Singer actually announces, "I just yesterday signed a deal to do an "X-Men: First Class" origins picture, which is kind of cool. I'm very excited." I've yet to see the trades confirm this, but since "Avatar" and "X-Men" are both Fox properties, I could easily see the whole thing being planned.