It isn't what Nancy Reagan is wearing that fascinates first family-watchers these days, but what the president is wearing--in his right ear.

The first lady came back from Arizona Sunday night with what the White House described as "a very long list" of people wanting to know where to buy a hearing aid like the one the president wore in public last week.

At an Association for Retarded Citizens dinner Saturday night in Mesa, Ariz., where Mrs. Reagan accepted a lifetime achievement award on behalf of her mother, so many people asked who manufactured the device that she wrote down their names and addresses and promised to get back to them, according to Sheila Tate, the first lady's press secretary.

"The president's decision to wear one did more for the hearing-impaired in this country than anything we've done in the last five years," says Michael Jones, marketing vice president of Starkey Labs Inc. of Minneapolis, which custom-made the unit in a hush-hush project that only a handful of technicians knew about.

Considered to be the pioneer in developing the device, which weighs a scant two ounces and fits almost unnoticed into the ear canal, Starkey made the president's without charge. It would retail for between $900 and $1,100 but was donated by Starkey and Burton Associates of Santa Ana, Calif., which ordered it.

"I'm sure the president could afford it," said Jones, "but we felt that his endorsement of amplification would do more to educate the public than anything that's been done to date."

Jones optimistically calls the president's action "a first step in bringing hearing loss suffered by an estimated 15 million to 20 million Americans out of the closet." From the standpoint of a marketing campaign, Jones is equally optimistic.

"I couldn't have bought it," he says.