While Lola hit a lot of the high points, there were a few misunderstandings that maybe I can clear up.
For example--Mental retardation isn't a disorder that "co-exists" with other disorders, such as Down's Syndrome. MR is one of the SYMPTOMS of Down's, that makes it a Syndrome to begin with.
(A Syndrome is a grouping of symptoms that occur so frequently together that they become a Named Syndrome, such as Down's Syndrome...)
Cerebral Palsy and other severe orthopedic impairments are similar, in that they involve so many of the body functions that MR is sure to follow as a function of the brain. Autistic children are not always MR, but a lot of them are. They are a group of students all unto themselves.
There are many factors that cause MR, such as prenatal drug abuse, lack of prenatal care and malnourishment, shaken baby syndrome, etc.
IDEA tried to classify MR so that parents and educators would have an easier time in placements, but that kind of backfired. The old classification still works: with an IQ of 100 being average, 2 standard deviations of 15 below the mean begins the classification of MR.
The term "developmental delay" applies to younger children so that they will have time to develop their languange and social skills before being improperly placed, and to provide services for the very young child which IDEA had not done until recently.
I think, to answer your specific question, that the issues in the definition and classification are still the same after all these years.
1. Once a child has been "labeled" MR, have their social and vocational opportunities been taken away? Should this be true?
2. If school systems when to full inclusion, with no pull out special education services, would the stigma of special education be taken away from these students?
3. Should parents have the right to refuse cognitive testing that would show a child's cognitive ability (i.e. IQ classification and MR) if it is likely that he will be MR?
(My opinion? No they shouldn't be able to. If a child has Down's, he is going to be MR. He needs the special ed. services, and the teacher and the school will be remiss in not providing them. The parents are being neglectful by not allowing the testing, and the school should take them to court under due process to obtain the right to test the child and obtain proper placement. This actually happened at a school I was in.)
Those are the issues that come up all the time. They are not likely ever going to change, either. We are dealing with humans and their children, after all.