Dr. Laura, how do you correctly use the N-word?

Recently Dr. Laura Schlessinger had a meltdown over use of the N-word on her nationally syndicated radio talk show. After some of her sponsors quit and she was forced into an apology, John King tried to get up a discussion about use of the word on CNN without much participation. There was some confused talk about First Amendment rights and double standards, but no one seemed really sure what to say. Before the story completely dies, here's a question about it: How "black" do you have to be to use the word without getting slammed?

It's clear that if you're "black," saying the N-word in the United States is completely okay, but if you're "white" it's not. This is something Dr. Laura pointed out on her show. You can hear the word uncensored from black performers on HBO and characters in black-produced films, indicating that this is completely politically correct. But if a “white” person uses the word, there are immediate demands for apology and for the person to be fired from their job. Clearly for anyone other than a black person to use the word is a serious error, and even unrelated but similar sounding words can cause someone to lose their position. In January 15, 1999, David Howard, a white aide to Anthony A. Williams, the black mayor of Washington, D.C., used the word "niggardly" (meaning stingy) in reference to a budget and was forced to resign.

Presumably one could go by appearance. However, studies have shown that appearance isn't really a good indicator of African genes. In the fifties, for example, one of five "white" Americans was found to have African ancestry. So, what criteria do you need to meet in order to be politically correct in using the N-word? Is there some measure of skin color? Of African genes or ancestry? Is dark skin enough, or does one have to have recognizably African features? Is President Obama black enough, for example? Shouldn't blacks politely inquire of someone's racial background before making a judgment that use of the N-word is racist? Or is “black” in the United States just a socio-political group that uses the N-word to set themselves off from the rest of America?