Back in the day when someone did something stupid you could call them retarded, however nowadays that is seen as a major faux pas, and it's heavily fallen out of favor. For a while there was no real replacement, but lately I've noticed more and more that "autistic" is stepping up to take its place. I mean I think the *autistic screeching* meme is the biggest example of this. Whoever made this meme couldn't say "retarded screeching" so they had to go with autistic.
My problem with this is that using autism as a substitute for retard is just factually wrong. Autism is a spectrum...and most of the people who have it aren't at the level of "autistic screeching", not even close. For a lot of people autism basically means #1 You have trouble interpreting social cues and #2 you have extremely specific and intense interests. Most people you meet who have autism you wouldn't know they have it.
But I think more to the point is this...if you are using the word to reference severely autistic people then there is literally no difference between using the word "autistic" and "retarded"...people have just basically found a new way to say the same thing, but it doesn't have the social stigma around it yet so they feel fine saying it. My advice? Just don't use it...because if you're uncomfortable calling someone "retarded" then logically you should have the same moral qualms about calling someone "autistic".