It was that kind of State Dinner at the White House last night: The prime minister of Canada broke step with his host to talk to a reporter, his wife raised eyebrows by showing up in a short dress, and the President's 9-year-old daughter Amy again brought a book to the dinner table.

All that and George Washington's birthday.

"I'm always a little bit moved and perhaps intimidated when I'm in the White House," Prime Minister Pierre Elliott Trudeau said during after-dinner toasts before a formal black-tie crowd of 100.

Earlier, Trudeau broke away from his host's side to talk to a reporter standing behind a velvet rope after he and President Carter descended from the second floor family quarters into the Grand Foyer.

"I love coming into that music," said Trudeau to the reporter.

The music was "Hail America," and it signaled and accompanied the descent of Jimmy Carter and his guest down the staircase to the East Room where guests awainted the second State Dinner of the Carter Administration.

It was one of two ceremonial selections traditionally played by the Marine Band led by Major W. D. Rusinak, who was retiring last night after 20 years as conductor. The other, "Hail to the Chief," was preceeded by "Four Ruffles," according to Mrs. Carter's press secretary, Mary Hoyt.

"That's the way it was done tonight --Hoyt, laying to rest once and for all a mini-flap about whether "Ruffles and Flourishes" had been played at the Carter's first State Dinner a week ago.

If Trudeau's spontaneous comment about White House music caught the press corps by surprise so did Margaret Trudeau's dress. It was white, appeared to be made of wool and was short and below-the-knees in length. The other women guests as well as the hostess wore floor-length gowns.

Amy Carter was supposed to have been seated with her parents and the Trudeaus at one of the 12 round tables for eight in the State Dining Room. However, she apparently asked to be seated with her sister-in-law, Annette Carter. Her book was entitled "Charley and the Glass Elevator."

The Carter family, as at the dinner for the Mexican president and his wife a week ago, was well represented --last night by son Chip (son Jeff stayed upstairs studying), as well as Ruth Carter Stapleton, the President's sister, and Mrs. W. J. Dolvin, the President's aunt.

In addition, the guest list drew upon early Carter supporters, less than a dozen congressmen, including two former presidential hopefuls, Sen. Henry Jackson (D-Wash.) and Edmund Muskie (D-Maine), and a few people from the artistic world. In a list of capsule biographies the White House released on some of the guests, the information was often humorous and rather off-handed. Harry Belafonte was described as (U.N. Ambassador) "Andy Young suggested: personal friend of Trudeau."

Alice Mason, an early Carter supporter, was listed as "a very interesting person -- is a numerologist and astrologist;" Louise Nevelson, the sculptor, as "very-well respected;" Phil Walden, the president of (Capri) corn Records, as "raised a great deal of money for the campaign;" and Oscar Ledford, of Franklin, N.C., as "profession unknown -- but these are really 'just plain folks.'"

Noting that yesterday was the official observation of George Washington's birthday, Trudeau told an anecdote about a Philadelphia newspaper that had bitterly criticized the administration of the first President. "You don't have to seek solace in that remark," Trudeau said to Carter.

He then replied to Carter's remark about a recent Gallup poll that found Americans choosing Canada as their second favorite country after the United States. "But if I'm ever in trouble politically . . ." Turdeau said laughing, then added that he knew where he could try another political career.

Trudeau also said that in the past, the United States and Canada had been "the hope of the new world," and that today, "We are the hope of the Third World."

In his toast, Carter said that both countries shared a "determination not to be dominated or pressured."