Columbia Pictures' megabuck comedy "Ishtar"--still in post-production and lumbering toward its May 15 national release date--is the talk of the town after New York magazine outlined the $40-million film's production delays and on-set peccadilloes. Among the latest rumors: the Elaine May film, starring Warren Beatty (who also produces) and Dustin Hoffman, was a candidate for the Cannes Film Festival's prestigious closing slot. A Columbia spokesman pooh-poohed the notion Wednesday, calling it "highly unlikely," since the Cannes fete closes the last week of May, well after the film's scheduled release date--which the spokesman insisted is very firm. Originally set for release last December, "Ishtar" long ago surpassed its initial $27.5-million budget. Location hassles and May's reported perfectionism caused a scheduled 14-week shoot to balloon into a six-month endurance test.