Last year, a recovery in housing prices seemed to be on track. But analysts now say that that improvement was juiced by home-buying tax credits that have now expired. In addition, unemployment has remained stubbornly high and millions of Americans are still at risk of foreclosure. A second slide in home prices would act as a drag on the economic recovery — and stand in sharp contrast to other downturns. During the real estate crash of the 1990s, for example, prices slowly but steadily rose from their bottoms. "It is going to be a rocky bottom, where we bounce around," said Stan Humphries, chief economist at Zillow.com, a real estate information site. And, after that, "it is likely that real estate appreciation won't keep up with inflation." The Case-Shiller Index, released Tuesday, showed that nine cities — Atlanta, Charlotte, Chicago, Detroit, Las Vegas, Miami, Portland, Seattle and Tampa — hit fresh bottoms, falling to the lowest levels since prices peaked in...