Halston wonders if her long dress wasn't lost at the cleaners.

Bill Blass asked if maybe she's got great legs and perhaps wanted to show them off.

But whatever the reason, New York designers last night frowned on Margaret Trudeau's choice of a knee-lenght dress to wear to last night's black-tie dinner at the White House. The wife of the Canadian Prime Minister, Mrs. Tradeau, who is as comfortable in blue jeans as French-designer labels, apparrently turned heads when she wore a short dress to a formal dinner in Toronto last week.

Not since Joan Kennedy appeared in a silver mini-dress at a Nixon black-tie congressional reception in March 1969 has there been so much attention paid to hemlines at the White House.

But in the late '60s, particularly in the Nixon White House, dressing up for formal occasions was de rigueur.

For the Carters, the emphasis has been on informality even for state occasions.

"There is so much stress on informality with the Carters," points out Molly Parnis "maybe this is what we will have to expect. But I think this is going too far."

Bill Blass, who calls himself a great exponent of the short dress for just about all occasions except the White House, said, "Maybe she did it out of deference to the Carter informality."

Added Halston, "I think it's curious and I feel it's a bit disrespectful to call that kind of attention to yourself at such an occasion."