Robert Motherwell, the noted painted, declined to serve on the Task Force for the Arts and Humanities, citing an unwillingness to cut funds from the arts as his reason . . .

Hot Rumor from the Cannes Film Festival, winding down its first week, has the all-time flop "Heaven's Gate" as a favorite for the grand prize, the Palme D'Or. As of last week, the disaster had grossed $689,872 in three weeks of release (compared with "Friday the 13th, Part 2," which had grossed more than $3 million in two weeks). Not that box office success is a measure of a film's worth. And besides, the French consider Jerry Lewis movies great art. Another strong contender for the Palme D'Or is the Polish film maker Andrzej Wajda's film about the Gdansk strike called "Man of Iron" . . .

It is am axiom that best sellers, regardless of their suitability for the screen are ultimately made into movies. No surprise then that the spy thrilled "Gorky Park" by Martin Cruz Smith, atop the best-seller lists, will soon be a movie. Producers Howard Koch Jr. and Gene Kirkwood have already signed British playwright Harold Pinter ("The Caretaker") to write the script and British director John Schlesinger ("Midnight Cowboy" and "Darling") to direct. To star? Koch and Kirkwood say they would like Al Pacino. Pacino is currently starring off-Broadway in a revival of "American Buffalo" . . .

Other film stars expected to tread the boards include Christopher Reeve, this summerat the Williamstown Theater in Williamstown, Mass., and Amy Irving, of "The Competition" fame, replacing Jane Seymour on Broadway in "Amadeus". . .