Michael When I was growing up, I usually chose one of two costumes for carnival: Either Cowboy or Indian. My circle of American friends has confirmed that there used to be similar customs in rural America as well, but that since roughly the 1960ies, it has been considered politically incorrect to dress children up as Indians, because the displacement of the native American population is considered to be one of the darkest chapters in the country's history.
Also, it would be considered an absolute no-no to dress up as a black person, as Americans generally don't condone joking about a person's ethnicity. An African American colleague at work once told me laughingly that while he was on vacation in Sweden, he once rubbed his eyes in disbelief upon noticing a group of partying locals with their faces painted black while wearing Afro wigs.
Recently, a local news show in San Francisco aired a segment on a poster campaign titled "We're a culture, not a costume", run by a third-tier university somewhere in the U.S., just in time for the upcoming Halloween celebrations. The posters depict young people making sad faces and holding pictures into the camera, showing Halloween costumes making fun of the person's cultural background. A Japanese girl holds up a photo of a carnival geisha, and a young middle eastern man shows a photo of a party goer dressed up as a suicide bomber holding up a switch. I don't know, I find it somewhat excessive to get all riled up at jokes like this, but we're living in a free country. As long as I don't have to hold up a picture of a Lederhosen-wearing Bavarian mountain man and make a sad face, that is.