Woody Allen made a shy but momentous appearance at the after-party for his Cinema Society screening of “Midnight in Paris.” The room at the Soho Grand almost fell silent Tuesday night as the director arrived with wife Soon-Yi, shook a few hands, awkwardly posed for pictures, and sped out the door within five minutes of arriving. Michael Sheen, who stars in the film, told us, “Woody doesn’t go to a lot of parties. He’s here because he really believes in this film and wants it to be a success.” Sheen — who is about to star in “Hamlet” in London — and his “Midnight” co-star Rachel McAdams confirmed they’re a couple, having stepped out together for the first time in public on the red carpet in Cannes last week. A glowing McAdams was showing off a sparkling ring on her left hand, but told us, “No, it’s not an engagement ring,” laughing, “I’ll have to talk to Michael.” Also at the Thierry Mugler-sponsored party: Patricia Clarkson, Valentino, Jimmy Kimmel, Sofia Coppola, Lou Reed, Julian Schnabel and newly engaged Nancy Shevell on a night out with girlfriends.