Kristin Cavallari won't divulge whether she will be joining MTV's The Hills, only saying that "it's a great show." But she's already partying like the cast – celebrating her 21st birthday in Las Vegas. The former Laguna Beach star marked the rite of passage at Luxor's LAX nightclub, where she danced to Soulja Boy and hung with friends. "My first official drink was Grey Goose and soda," she ...