Okay, so the title is a little sensationalist, but I was reading an article about a school in Ohio targeting racist Halloween costumes. Here is one of the articles:
Clickie
I think most people would agree that dressing up as an "illegal alien" or a "terrorist" is pretty blatantly awful, but in comments of one of the blogs I read, they were debating about the geisha costume. Some people didn't see how it was a problem for non-Japanese people to wear it as a costume, as it's a real thing. Other people said that it was racist. Can a Korean person wear a geisha costume? Can a white person ever wear blackface, even if it's that their all-time favorite movie character is Jules from Pulp Fiction?
Personally, I think if the intent is to simply represent someone of a different race or culture without making fun of it, it should theoretically be okay. It'd be nice to see the racial boundaries simply fall away. In reality, though, it may be awfully hard to convince others what your intentions are.