While much of America worries about foreclosure, John Paulson, who made $3.7 billion shorting subprime mortgages, has plunked down $41.3 million for a Southampton estate. But, amid whispers about which Wall Street casualties will lose their summer spreads, the market for properties below $10 million is grim. Michael Shnayerson checks the real-estate temperature of the country’s most celebrated summer retreat to see if its mere mega-millionaires are about to take a cold dip.