When I was younger, I had a best friend whose sister was mentally challenged due to a brief cut off of oxygen at birth. On the outside, she was a happy, well-adjusted pre-teen. On the inside, she had the mind of a 5 year old.
She had a speech impediment, which made it difficult to understand her. However, over time and countless visits to my friend's house, I came to understand her speech better. Eventually, I was able to communicate with her without assistance (for me, not her) and she grew to like me a lot since I treated her as I would any other girl her age. When I looked at her, I saw my best friend's little sister, not the girl her classmates called "The Retard."
At an early age, I learned not to use that word casually as other kids my age did. I learned that it was inappropriate to say, "That's so retarded" or "So-and-So is such a retard." Whenever I hear someone use that word (even now), I can't help but wince. It's extremely offensive to me. Imagine how offensive that word is to someone who is mentally challenged.
By the way, in addition to the derogatory use of 'retard', I also can't stand when people use the word 'gay' as a synonym for 'lame' (i.e. "That's so gay!").
Probably triggered by Obama's Special Olympics gaffe on the Tonight Show a few weeks ago, the Special Olympics has started a campaign to rid the world of the inappropriate use of the word "retard" and its cousin "retarded".
And frankly, I'm all for a little bit of education and awareness. Especially if the goal is to eradicate ignorance.