Nancy Reagan caused a near sensation the night she wore it in Peking at the reciprocal dinner she and President Reagan gave for Chinese Premier Zhao Ziyang. High-necked and close-fitting, with deep slits up the side, the red satin brocade cheongsam of the type Suzie Wong made famous was a state gift to the first lady from Madame Lin Jiamei, the wife of China's president.

Tonight, Mrs. Reagan will wear it again, this time at the state dinner she and Reagan are giving for President Li Xiannian and Madame Lin. As a symbol of the importance the Reagans are attaching to the Chinese leader's visit, give it a 10.

The Chinese are big on symbolism. That Reagan is going ahead with the dinner so soon after his surgery for removal of a cancerous tumor indicates to the Chinese that the American presidency is far from weakened by his illness, Chinese sources say.

Even if Reagan's appearance will be abbreviated to conserve his strength, there's nothing abbreviated about the guest list. On it are Elizabeth Taylor, Robert Mitchum, Walter Cronkite, I.M. Pei, Mary Lou Retton, Bette Bao and Winston Lord, Susan Eisenhower, John and Barbara Hersey, Anna Chennault, Washington Ballet's Choo San Goh, American Ballet Theatre's Lincoln Kirstein and designer Valerian Stux-Rybar.

And if titled nobility seems just a little odd on tonight's guest list, chalk it up to liberte', egalite', fraternite' American-style. In this case the title belongs to France's Princess Chantal, daughter of Henri Bourbon-Orleans, sometimes called the Count of Paris and the man some Frenchmen think would be king if France had a king.

Since even the offspring of pretenders to thrones can't pretend they aren't curious about how life at the top might have been, the princess accepted the Reagans' invitation to dinner with alacrity. And since dinner guests are usually too busy seeing each other to look around the White House, arrangements have been made for Chantal to return later this week for a tour. A VIP tour, naturellement.