I think the whole business of which words can be used by whom is fascinating...my own gut feeling is that the "n" word is just inherently offensive, it has so much unpleasant baggage attached to it that it's hard to see it ever being effectively "(re)claimed".
Most African Americans of my acquaintance find it distasteful and wouldn't ever choose to use it; on the other hand, if some black people want to take it back and reinterpret it for their own use I'd have to say they have every right to do so.
It's sort of like some feminists, who feel that the "c" word can be rehabilitated and reinvented as long as it's only used by *women*, ie, the language of the oppressor can only be used by the oppressed. Again, it's not a word I personally choose to employ (there are so many others!), and it seems to me that by continuing to include words that have been used to debase and devalue people--ANY people--we participate in continuing that debasement and devaluation.
Words *do* have power, and it seems that Michael Richards has learned this particular lesson in a very hard (and public) way; it's pretty doubtful he'll be including *that* word in any future appearances, should he be so fortunate as to land any.