There are some nice examples in Leah Rozen, “Hey, Ryan, Talk to the Dress“, NYT 2/10/2010:
RYAN, Ryan, Ryan. It’s Journalism 101: who, what, when, where and why, as in, “Who are you wearing?” […]
Susan Kaufman, editor of People StyleWatch, said she lost it when Mr. Seacrest didn’t immediately quiz an elegant-looking Sandra Bullock — who would later win as Best Actress — about her shimmery frock (by Marchesa). “I’m screaming at the TV: ‘Ask her who’s she wearing!’ ” Ms. Kaufman said. “I was so angry, my husband was laughing at me.”
Different versions of the same question come up in Elizabeth Wellington, “Mirror, Mirror: So, E!, whatever happened to ‘Who are you wearing’?“, Philadelphia Inquirer 2/10/2010:
Erica Salmon, president of Mullica Hill-based Red Carpet MVP (formerly the Fantasy Fashion League), wholeheartedly agreed. In the online game that mirrors fantasy football, points are awarded for how many times designers’ names are mentioned by the media during red carpet season. The Oscars are the championship game.
“We can’t just use E! anymore,” said Salmon, who has been forced to use InStyle.com because E! didn’t give her players enough information. “Sometimes I wish I had a direct line to Ryan’s mike and I’d say, ‘Dude, please ask who are they wearing?’ ” Salmon said. “That’s what we need to know.”
And dozens of other news outlets are discussing the same question, as they’ve been doing more and more often over the years (about 35 times more often than “Who are you reading?”, apparently). The earliest example in Google’s news archive is “Red Carpet, Big Smiles, Tight Security”, San Jose Mercury News, 3/26/1991:
Peggy Lipton, Dianne Wiest and a buxom Egyptian model named Kelli fielded the evening’s most pressing question: “Who are you wearing?”
However, a search in ProQuest’s Historical Newspaper Archive turns up Genevieve Buck, “Chili, Bud kick off Chicago fashions”, Chicago Tribune 4/11/1984:
“Who are you wearing?” turned out to be the game of the evening.
So (pending Ben Zimmer’s discovery of a citation in Elizabeth Barrett Browning’s diary) it appears that this is an expression that emerged in the 1980s in a general fashion-show context, and became a touchstone of Oscar-night reportage at some point over the past decade or two.
It also turns out that Who Are You Wearing? was a reality TV show, as of a couple of years ago. Somehow, I managed to miss it.
[Update: more here.]