Some of the nation's frothiest housing markets are at increasing risk of price declines, says a recent survey from PMI Mortgage Insurance Corp. The PMI Risk Index is based on economic activity and other conditions that PMI thinks are predictive of home-price declines over the next two years. Factors used to derive the index include home prices, employment conditions, and the affordability of homes. The index names Boston as the area most at risk for a decline, while the Nassau/Suffolk County area, in suburban New York, is a close second. Safer areas include Detroit; Pittsburgh; Memphis, Tenn.; and Indianapolis.