There's a tradition in England - I don't know how old - of abusing red-headed people.  It's a genuine prejudice in England.  From this facebook page:

'Ginger' in England basically is like saying:

"Look there's an ugly, smelly, no friends, socially unacceptable, negative, aggressive, angry, violent, unclean, nasty, non boyfriend material, low self esteem, unattractive, social misfit, nerdy, moron, low education, non human...etc etc etc"

The term 'ginger' didn't become 'mainstream' just because of that South Park episode, I was being shot at, having acid thrown over me, stabbed, headbutted, punched, spat on, kicked, dehumanised, singled out, socially excluded, avoided, belittled, character assassinated etc since I can remember and to be fair I found that treatment was at its peak years before that South Park episode was even thought up.

This spread to the US in 2005, when Cartman tried to incite violence against redheads in a South Park episode with "Kick a Ginger Day".

What's interesting is how this meme is spreading in the US: As humor.  This meme is promoted by sites like CollegeHumor.com and MyLifeIsAverage.com, which mine it as a source of ironic humor.  The Cheezburger Network is pushing ginger-hatred almost as aggressively as they push pedophilia as a fount of humor.

Are humans capable of, collectively, keeping real and humorous/ironic racism separate?  No, they are not. What South Park "kicked" off as an ironic commentary on racism is becoming actual racism.

One clue that you're going too far in your ironic humor is when you start finding the real thing funny.

Do humans have an instinctive need to bond over shared prejudices?  Is combating racism a game of whack-a-mole, in which society invents new prejudices to replace the ones being taken away?