AOG
I've been thinking about this since the last time you raised this topic, and I think I can explain why it is deemed (by some) as an acceptable word for (some) black people to use, but not by non-blacks...
In my opinion, it's a reserved word along the same lines as a nickname you may have for a close friend or family member.
Let's say you have a brother who underwent an operation on his face which left him with a scar...amongst the family, he may have the nickname 'Scarface', which he accepts because it's family, and he knows that there is no malice in the sobriquet.
However, if your family went out to dinner one night and the waiter referred to your brother as 'Scarface', it would be completely inappropriate.
If you think of the 'N' word along those lines, it makes sense with regards to its status as a 'reserved word'.