I have an uncle with Downs, and I still call my friends --and myself-- a spastic whenever we fuck up. The thing is, when I'm calling someone a spastic I'm not saying, 'lol let's kill disabled people' because the term is now removed from people with learning disabilities and accepted as a colloquialism for stupidity.
The thing that annoys me about this is I guarantee most the people who have a problem with words like 'retarded' wouldn't hesitate to call someone 'dumb,' even though that used to be a medical term for those unable to speak. Just because it's been out of the medical lingo for so long and is now accepted as common parlance, no one bats an eye at a film called Dumb & Dumber, but Retard & More Retarded? There'd be fighting in the damn streets.
There is no such thing as an offensive word, context is what makes it offensive. Would I say spastic around my uncle? No, but I wouldn't talk about hymens around my gran, either. I say 'what up niggas' to my friends, as a term of endearment, even though we're all white, but I wouldn't say it around black people because it could be interpreted as a racial slur.
That being said, if I had a black friend who knew me well, I know he probably wouldn't mind me saying it, because he would know there was no malice attributed to it.
I admit people are too quick to use the whole 'it's political correctness gone mad' line when they want to get away with saying something offensive, but it doesn't mean we should tar everyone with the same brush and assume that we all use these words to incite hatred or get a reaction.