It's no secret that the volume of distressed residential properties is weighing heavy on U.S. housing markets and prolonging any meaningful recovery. Of even greater concern is the industry's growing backlog of homes that need to be liquidated and resold but have yet to make their way to the market. Standard & Poor's has just released a new report in which it estimates that the principal balance of this shadow inventory now stands at $460 billion and will take the industry about 41 months to clear.