Cartoonist Garry Trudeau has unwittingly caused more problems for the U.S. Postal Service. Postal employees have been told to keep their eyes open for letters with stamps bearing characters from the "Doonesbury" comic strip. The stamps are from the "1990 Doonesbury Stamp Album," a book released last month by Penguin, and a few have slipped through the automated canceling machines. In May a Sunday Doonesbury strip contained drawings resembling postage stamps, along with instructions to use these "stamps" in protest of proposed rate increases. While Trudeau meant for those stamps to be used, he said yesterday that he had no such plans for the most recent batch: "There was no subversive intent. These were intended purely for decoration and for kids to collect. I can't imagine anyone using these any more than they would those National Wildlife or American Lung Association stamps." Art Shealy, a spokesman for the Postal Service, says that he doesn't think Trudeau was telling people to deliberately use the ornamental stamps in place of genuine ones, but that the Postal Service would investigate any such use. "Is it illegal? Yes. The percentage of stamped mail we carry is very small compared to metered mail, but we still want people to know that it's fraud." Profits from the Trudeau book are to benefit the Literacy Volunteers of New York. Glasnost Goes to Harvard Sergei Khrushchev, the son of former Soviet premier Nikita Khrushchev, will be one of seven fellows at Harvard University's Institute of Politics this fall, said institute Director Terry Donovan. The Soviet engineer, who is the author of the recent book "Khrushchev on Khrushchev," will teach a seminar as well as research another book. He and Melor Sturua, former Washington bureau chief of the newspaper Izvestia, will be the first Soviet fellows at the institute, which is part of the John F. Kennedy School of Government. Redford's Casting Call A Hollywood watchdog group for American Indians has questioned Robert Redford's decision not to cast an Indian actor in the leading role of a Navajo policeman in his new film. Lou Diamond Phillips has been selected to play Jim Chee in a film based on New Mexico author Tony Hillerman's novel "The Dark Wind." Redford plans to begin filming on the Navajo and Hopi reservations Sept. 10.

Bonnie Paradise, director of the watchdog group the American Indian Registry for the Performing Arts in Hollywood, Calif., said she will meet with Redford and Phillips within the next three weeks. Paradise said that Phillips claims he is part American Indian but has not given the registry any documentation as requested.

"We would have really liked to have a Navajo actor play Jim Chee," said Sammye Meadows, the Native American liaison for Dark Wind Productions. "We spent enormous amounts of money traveling to powwows. ... We didn't find the actor."

The Pop Star Plague Is there a trend here? Marie Osmond is the latest female performer to fall ill while on tour, after Madonna, Sinead O'Connor, Janet Jackson and Joan Armatrading each suffered health problems on the road this summer. Osmond collapsed onstage Wednesday night at the Stark County Fair near Canton, Ohio, and was taken to a nearby hospital, where she was treated and released yesterday morning. "She had a severe stomach flu," said her manager, Karl Engemann, but Osmond didn't want to disappoint her fans at the fair and so had gone on with the show. About 45 minutes into her set she grabbed a stool, said local reviewer Dan Kane, "and fell, boom, down to the floor, and they carried her offstage." The Friedheim Finalists The four finalists for this year's Kennedy Center Friedheim Awards were announced yesterday by the competition's jurors. The awards for new American orchestral and chamber works will be made Oct. 28 at the Kennedy Center after a concert at which all four pieces will be performed. The works and their composers are "The Rauschenberg Variations," by Frederick Bianchi of Cincinnati; "Common Ground," by Daron Hagen of New York; "Veils and Variations for Horn and Orchestra," by William Kraft of Altadena, Calif.; and Concerto for Cello, Piano and String Orchestra, by Ralph Shapey of Chicago. The first-prize winner will receive a cash prize of $5,000; second prize will be $2,500; third prize, $1,000; and fourth, $500. The annual awards were begun in 1978 to recognize outstanding new works by American composers, and are supported by the Eric Friedheim Foundation. Johnny Depp's Jumping Career Actor Johnny Depp, star of Fox TV's "21 Jump Street," has signed a multi-picture development deal with 20th Century Fox for projects to produce and star in. He starred in the John Waters movie "Cry Baby" this year, and will costar with fiancee Winona Ryder in "Edward Scissorhands," scheduled for the Christmas season.

-- Compiled from staff and wire reports by Eric Brace Chuck Conconi is on vacation.