William Ackman is about to take a bet on two of the hardest-hit sectors of the economy: housing and commercial real estate development. Ackman, who runs hedge fund Pershing Square Capital Management LP, is one of the lead investors in a new company that Chicago-based General Growth Properties Inc. is spinning off as part of its bankruptcy reorganization. He also will be chairman of the new firm, a money-losing collection of partially built malls and housing developments.