There's something in the airbut don't inhale. Marijuana use is growing like, well, a weed on the big and small screens. Take Aug. 3's Sex and the City, in which Carrie and her buds smoked up. And, despite its title, Mandy Moore's How to Deal didn't pack instructions on selling drugs -- but her character's grandma sure knew how to pack a bowl. Even America's favorite potheads, Cheech and Chong, have signed on for another joint venture. ''It's an amazing cultural shift,'' says Allen St. Pierre, exec director of the NORML Foundation. ''It's the aging of the baby boomers and the generations that follow.'' Put that in your pipe and smoke it. And pass the Doritos.