Cultural appropriation, as far as I can tell, is a relatively new concept used by those who traffic in identity politics to shut down raw speech and expression of those who use mediums traditionally endemic to other cultures than their own, especially minority or dispossessed ones. I've read the thread, but I feel the need to define it to say this:
It's a pernicious concept. It decries cultural sharing, as Maurile pointed out above, and could actually be called the new autarky of the cultural identity politics left, in that it is tribalistic and potentially nationalistic. I think it implicates raw free speech and raw free expression, limits talent to a select few, and puts otherwise earnest people on the defensive in ways that implicate very sensitive things.
To use the example Tobias did: We all know Elvis "appropriated" Chuck Berry, but Berry was popular in his own day. It's authors like Nelson George and others who complained about the popularity of Elvis, but Elvis was a product of a time a lot more racist than the one we have today. We also know Eminem "appropriated" hip hop and rap, and he even admitted it. But here's the thing: They were two wildly popular and respected entertainers in their own right, both by fans and colleagues alike. They might not have deserved their wild popularity, but many people will tell you that they were the best in the game at the time. And that's the rub. The new "cultural appropriation" warriors won't allow for the most talented among us to use form and medium to express ourselves. They'd rather shut people up than take Q-Tip's approach, which was, "You can be white and groove but don't crap the roll."
People also seem to be confusing exploitation with expression and speech and form and medium. Exploitation is a different matter entirely. When Mario Battali (Batali? I read Heat by Buford and Batali looks right) apprentices in Italy or Spain, he's following in a tradition of people and chefs who have done this. When people travel from Portland to Mexico to find out how to make authentic burritos, they're not stealing anything. They're following in a tradition of cultural respect and cultural sharing. We've gotten to the point where I hear people discussing Justin Bieber's dreadlocks in settings where they're angry he's grown them. What happened to laughing at that like the Ras Trent of yore? Exploitation happens for many reasons, not the least of which is intellectual property laws. Berry Gordy was one of the worst violators and labor exploiters in the music industry, yet nary a peep from the likes of a George or a Mos Def. Diddy absolutely hosed his artists in their recording contracts. Again, nary a peep (well, the Lox made a peep, but that's for another day.) Anyway, my long point can be condensed as such: Exploitation of labor happens everywhere, cultural appropriation seems to seek to head that off by limiting cultural sharing.
Cultural appropriation is, as Redmond said, maybe not the sign of the idiot, but the sign of somebody who is limited in worldview, outlook, and wants to limit our natural right to speech, expression, labor, you name it.
It's sinister.
Also, when I say speech, I say it in the Tocquevillian sense, as always.