Or "stunt person"? I don't know how to refer to her. Anyway, a ballerina performed the dancing in "Black Swan," and the magic of CGI put Portman's face on her body. This was clearly shown in a special effects video circulating the internet. However, the studio put the kibosh on that, squashing the video and asking the ballerina in question not to give any interviews about what she was actually doing on the movie. This raised some outrage from dance people, who were annoyed that the studio was pretending that Portman just worked reaaaally hard for a year and became a ballerina capable of doing a challenging ballet.
And then, Portman did not thank her dance double when she received her Oscar.
I don't think the etiquette question is about the studio so much. Obfuscation in the name of suspension of disbelief is par for the course. However, should Portman have thanked the woman who did the actual physical work when she received her award? Or is the ballerina in question not really an important part of her performance?
I'm inclined to think that Portman was not in the wrong for not thanking the woman. She presumably won the Oscar on the strength of her acting, not because the Academy was so impressed with her dancing. That said, it would have been kind to at least mention her.