What are we supposed to call the people we used to call "retarded"?

There is this campaign to end the use of the word "retard" or "retarded" to describe people with mental disabilities. But what are we supposed to call these people? Is removing "retard" from our collective vocabulary political correctness taken to an unhealthy extreme, or is it based on good intentions? I am not trying to deny their humanity, and I believe the "disabled" deserve more respect and better treatment from society.