After almost two decades of self-imposed semiretirement, pianist Van Cliburn is making a comeback. And the Grant Park Music Festival nabbed him for its 60th season opener to the tune of a reported $50,000. Never mind that Cliburn has never lived up to the early promise he showed following Moscow’s 1958 Tchaikovsky Competition. As the first American winner of that prestigious contest (at age 23), he was lionized as a conquering hero, and back home his recording of Tchaikovsky’s First became the first classical disc to go platinum. He also championed the ultraromantic concerti of Rachmaninoff and Liszt, but shied away from Mozart and Beethoven–as if realizing his own limitations and preferring the safety of a limited repertoire. At Grant Park, not surprisingly, he’ll reprise the Tchaikovsky, under the baton of the sure-handed Leonard Slatkin. If nothing else, the prodigal son’s return ought to please legions of fans nostalgic for the kinder, gentler Eisenhower era. Slatkin, formerly of Saint Louis and now of Washington, D.C., where he’s taken over Mstislav Rostropovich’s place at the helm of the National Symphony Orchestra, offers a more audacious program the next night, headlined by Carl Orff’s paganistic Carmina Burana; soprano Harolyn Blackwell, tenor William Watson, and baritone Vernon Hartman will perform with the Grant Park Symphony Chorus, which will dedicate the concert to the memory of its founder, Thomas Peck. Saturday, 8 PM (Cliburn), and Sunday, 7 PM, Petrillo Music Shell, Grant Park, Columbus at Jackson; 819-0614.