twatsrus - As you can probably work out from the thread, it's complicated. There's a kind of positive usage and a negative one. The positive one is tied up with the sociological concept of "appropriation", and basically comes down to the idea of an oppressed group adopting a term of abuse and taking the sting out of it by using it themselves in a joking sort of way. Note that typically, not everyone within that group will be happy with this - some may quite legitimately feel that the word is so bad that it can't be neutralised through humour/appropriation.
An example of this might be the recent phenomenon of "Slutwalks". The idea grew out of a male police officer (I think very senior one in Canada, IIRC) saying that if women went out dressed as "sluts" they brought sexual assault on themselves. A group of women started "Slutwalks" as a riposte to this (reminds me of the chant I remember from "Women reclaim the night" marches: "What ever we wear, where ever we go, yes means yes, no means no"). The point is of course that it is alright for the women organising/participating in these things to self-identify as "sluts" as an act of political satire - it's not okay for others to point at them and say "you're sluts." Basically, rule of thumb - if you're in the group and it's a private setting, and you know one another well enough to know each other's sense of humour - you can joke about it (like the poster upthread who mentioned that she and her brother had an ongoing private joke about papists/heathens). If you're not - common rules of politeness apply. And, re. the slutwalk thing, there were quite a lot of feminists who said "look, we can see what you're trying to do with this, but actually, that word is so offensive there's no possibility of using it ironically/reclaiming it."
So when two members of the same black socio-cultural background both choose to use the n-word in banter, that's their choice, but it's not an invitation for other people to join in, nor does it mean that all individuals from that background will think it's okay.
And of course the situation then gets massively more complicated when you take into account tensions between individuals from groups from different socio-cultural backgrounds who may exhibit racism aimed at each other - which seems to be the case here. Racism is still unacceptable when it's, say, black African vs. black Afro-Carribbean, or white English on white Irish descent (with the usual caveats that there's a big difference between an individual act of racial prejudice - for instance someone black calling someone white "honkey", and ongoing prejudice backed up by historical and social inequalities - someone white calling someone black the n word. Both are wrong, but it would be naive not to recognise that the latter has the potential to be much more damaging).