Don't feed the trolls. And in that way it is the same word as "stupid" or "idiot" or "crazy". The word has lost its meaning. People who use retarded now use it as a general curse/insult, not as a word with definition.
Not nearly as much as using the word nigger. In fact, AFAIK you'll get more a "why the heck are you using that word" reaction, so long as you aren't hostile in using the word. And I know you don't need my permission. I wasn't giving you permission. I was stating fact. You are allowed to curse on the internet.Absolutely, but the term still does refer to the mentally handicapped. And it draws its meaning from a comparison to negative stereotypes of them.
I think you'll still find people offended if you call them negroes, though.