How’s this for odds: If you have a house in Las Vegas, there’s a 58% chance you owe more on your mortgage than the place is worth. Nevada, of course, is ground zero for the real estate bust but it’s hardly alone in having truckloads of “underwater” homeowners. As of December, 19.8% of mortgage holders nationwide had negative equity in their houses, according to a new report by loan-tracker First American CoreLogic.