The dress was by Peter Soronen. It's the fourth gown by the designer the first lady has worn.
Soronen said he knew the corset-style gown, complete with lace-up back, was being considered for this lavish event that drew about 200 people, including celebrities Whoopi Goldberg, Eva Longoria Parker and George Lopez. But, he added, he didn't know for sure until she stepped out on the president's arm.
"In some ways, it's more exciting not to know and just wait for the photo," Soronen said.
"It's a clever, memorable, sophisticated choice," said Avril Graham, executive fashion editor at Harper's Bazaar. "It's one of my favorites that's she's worn. It's all so right, so picture-perfect right."
Graham said she particularly liked the cobalt color, which stood out against the red carpet and the White House background.
Deborah Lloyd, creative director of Kate Spade, says women are inspired by Mrs. Obama's fashion choices because they are "soft, feminine and modern."
"It's what a lot of women aspire to," she said, "and she shows it doesn't have to be a power lady of another generation."
The Soronen dress was adorned with a silver belt, a look the first lady favors, and had a hint of shiny lame under layers of chiffon. She wore her hair swept up in curls.
"The dress feels a little like the inaugural dress - Part 2 - but she looks very fresh with the color and her hair," said Katie Connor, fashion features editor at Marie Claire.
One-shoulder dresses are a bona fide fashion trend, but Mrs. Obama's always seem a little different because the shape is more like a strapless with a single swath of fabric that emphasizes her collar bone, Connor said.
Mrs. Zavala, meanwhile, wore a plum-colored gown with an Aztec-inspired square neckline bordered in blue by Macario Jiminez.
And orange dresses were popular among guests, including Longoria Parker in Reem Acra and Gayle King in Carmen Marc Valvo.
House Speaker Nancy Pelosi stood out in an ivory gown with matching jacket by Giorgio Armani.
Olympic speed skater Shani Davis was a little weak on the details of his fashion statement.
He said his green tie was from Men's Wearhouse and guessed his suit was, too. But then he checked the label and announced, "Ralph Lauren."
Marie Claire's Connor noted that New York-based Soronen has done several outfits for Mrs. Obama lately. There had been an expectation that she might wear a Latin American designer in keeping with the dinner's guests of honor, Connor said, "but you have to commend her for knowing exactly what works on her."
"I love the structured bodice that I do for her. I put it in a lot of dresses," said Soronen. "And then I drape and pin and stitch all that lightweight chiffon, and I love that look on her."
