This week began just as usual on the sleepy Yahoo Finance Web site where Southern Co.'s investors post their thoughts occasionally. A message or two a day, nothing exciting. Someone mentioned an upcoming dividend. Someone answered: Southern Co. "remains a safe harbor, except for the MIR fiasco, which still bugs me," the poster wrote, referring to the now-bankrupt Southern spinoff, Mirant Corp. A few hours later, investors burned in that fiasco roared in and hijacked Southern's placid message board. By Monday night, they had overrun the Yahoo message site assigned to Mirant's former parent. These were the same investors who...