My take on political correctness is that it is/was necessary to prevent minorities being offended by certain words/terms that were in common use amongst the white population, and preventing offence in this way can only be a good thing.
Unfortunately now we reach a stage where well-meaning (but IMHO misguided) mostly middle class liberals are trying to dictate to everyone else how they should think, by taking PC to rediculous levels. Please, will somebody find me a black person who finds the descriptive term for a chalkboard 'blackboard' offensive, as I've yet to meet one!
The problem is by taking political correctness to such ludicrous levels, whereby innocent phrases such as 'blackboard' or 'bah bah black sheep' are deemed potentially offensive, and whilst not banned, use of such phrases is discouraged, or taxi drivers urged not to display the St. Georges cross on taxis during football tournament (as occured in Wolverhampton, as apparently it might be deemed a racist symbol), many people incorrectly blame the minorities for this creeping onset of political correctness taken to extremes, and this is why political correctness is (IMHO) becoming a double edged sword that is beginning to hinder race relations as well as help in some respects.
Minorities become the target - people are outraged at, for example, the Christmas holiday break being called 'winterval' by Birmingham City Council (a move they made a couple of years back IIRC that was somewhat unpopular and has since been reverssed), believing it to be a case of ethnic minorities attempting to subvert or change the existing cultures and traditions of this country to suit them, and as such racial, religious and/or ethnic tensions are exacerbated as some elements of the white majority react with hostility to the petty changes they see taking place.
Yet the problem lies not with the minority population, I mean really, do most Hindus/Sikhs/Muslims/Buddhists or whatever really find 'our' celebration of Christmas so offensive? Do epileptics really get offended by the use of 'brain storm' rather than 'mind-map' to describe a way of setting out information? Do many black people get offended at the mention of a blackboard?

Apparently they do - these are (to me) some of the more silly ideas to come out of political correctness, but I use them to illustrate a point.
In my experience, no, most minorities are not offended by Christmas, the use of the word blackboard, and so on - but still small changes such as these are made in the name of political correctness that somehow manage to breed resentment amonst the majority population against the people they see behind the issue..
The media is most unhelpful in this respect, tabloids are keen to highlight such issues of political correctness but never point the finger clearly at who is responsible - liberals who think they know what does or doesn't offend people are the ones pushing 'political correctness' to such levels as exist today, not the minority populations political correctness originally set out to help.
All IMHO, I'm sure someone will disagree

but thats how I see the issue.