it would all depend on what the disability was. someone might have an arm missing, someone might be in a wheelchair, and then someone might be mentally handicapped. i have worked with a mentally handicapped student, and patience is the best tool to have when dealing with that. i just try to treat them as equally as i do others, but i would never be as tough as i am on the average student. plus, the child's father was just taking him to karate to have something to do with his son, and that just fine by me.
now with the physical side, you have to do some homework i think. analyze the disability, and see how it might actually be used to the persons advantage. i have seen people in wheelchairs use the chair as a weapon of sorts.
if a person doesn't have a hand or arm, the style needs to be altered. it's all about tailoring your particular art to what that person is able to do. one should be able to have enough understanding of the art to tailor it, if they don't they should be teaching, but learning.