"Twitter is the side project that took," says company co-founder Biz Stone, 35. "Now it's our chance to do something transformative." When I arrive at Twitter's headquarters on a recent morning, Jerry Brown is waiting in the lobby -- just another day at the world's hottest high-tech company. "It's pretty bizarre," says co-founder Evan Williams, 37. "At least once per day we look at each other and say, 'What the hell?' It's like we're living out the script of the ultimate start-up company story." But other than the familiar face of California's attorney general standing near the steel front door,...