Consumers are practically shrugging off the lowest mortgage rates in history. They have their reasons for hesitating to buy homes or to refinance existing mortgages, freeing up cash to spend: Eleven million residential properties are worth less than the mortgages on them. Nearly 15 million Americans are unemployed. Millions more are worried about their jobs; and still more are determined to cut their spending and pay down their debts, ignoring the siren song of cheap money.