Asperger's is already commonly considered to be a high-functioning form of autism; not sure if that's been reflected in the DSM before or not. If it has, then they're eliminating a redundancy; if it hasn't, then they're updating the manual to reflect something that most psychologists already agree on.
Is this something that happens regularly to terms that get adopted as common insults? Because I don't think "dumb", "lame", "idiot" or "retard" are in any modern medical manuals either.
I don't think that's what happened here but you're right; "dumb" has become "mute", "lame" has been replaced with a litany of different terms of varying specificity, "idiot", "imbecile", and "moron" have been replaced with profound, severe, and mild mental retardation, respectively. And while "retardation" does appear in the literature, my experience is that psychologists and special educators use the word professionally among one another but, as you might expect, don't like using it in conversation with parents or students.
But again, I don't think Asperger's has been reclassified because people are worried about its increasingly common use as an insult, I think it's because doctors and educators have agreed that it belongs in the autism spectrum and shouldn't be classified as a separate and unrelated condition. Their belief is that Asperger's sufferers will be better served, in terms of diagnosis and treatment, by being considered alongside other autists. (Cuts both ways, of course: there are presumably people who have been diagnosed with autism but not Asperger's who do in fact have Asperger's.)
That's the theory, anyway, and I think it's sound -- but we'll see how it works out in the coming years. My fiancee's going into special ed with autism as a specialty; she was a little surprised by the change but acknowledges it makes sense. We'll see what happens.