I don't think that anyone with any sense believes that Portman is a ballet dancer or could be one if she so desired, just as I don't believe anyone with any sense believes that Mickey Rourke, Sylvester Stallone, Mark Wahlberg or Hillary Swank are, or could be, boxers even though they have played boxers to acclaim in films and had to train physically for their roles.
And you always read about how hard they trained, and I'm sure much of it is true - they work hard just to be able to approximate for the screen the level of skill they're supposed to have. The fact that Portman didn't make a fool of herself is tribute enough to her training. atm711 made mention of Claire Bloom and Melissa Hayden - Chaplin made Bloom attend ballet class every day and work hard at it, and Bloom wasn't called upon to do half of what Portman did.
Well, they may not have much sense, but after the film was released people were contacting Covent Garden and NYCB to find out when Portman would be dancing in Swan Lake. I wonder, were there similar cases of people expecting to see Stallone in a title fight?
There's not much you can do about the [really] credulous. Had those people paid more attention to what Portman said in her interviews, they would have had no such expectations.
I suspect that ballet is still very exotic to much of the public, and unfamiliarity has something to do with it, as well. They see a lot more of sports.