Martin Sullivan has stepped down as AIG's CEO, and the company's board named chairman Maurice Willumstad his successor, insiders tell the Wall Street Journal. The insurance giant's stock has plummeted more than 50% since October due to subprime writedowns, and Sullivan is still embroiled in legal battles with his former boss. The new chief may start with a clean slate, but the state of American real estate remains daunting. (More subprime crisis stories.)