Around the early 20th century, the medical terms for people with various degrees of severe cognitive deficits were morons, idiots, and imbeciles (yes, this is true). Partly because those terms quickly became pejoratives, ~50-60 years ago, they were replaced with mildly, moderately, and profoundly retarded. Now "retarded" and "retard" are pejorative, so we're moving to replace that. On the other hand, idiot, imbecile, and moron are pretty much considered run-of-the-mill words for stupidity.
So, no matter how PC of a term we replace retardation with, there'll quickly be a derogatory term based off it. And "retarded" will fall back to being an ordinary word for stupid. I guess what I'm saying is, bring on "intellectually disabled" and let the schoolkids of America do what they do best and make fresh insults from it - we're takin' "retarded" back!