Housing got us into this mess, and now it’s threatening to yank us back in. That’s the news from the latest S&P/Case-Shiller Home Price Index, out Tuesday, which shows that home prices in 20 major cities fell in March to their lowest levels since 2003. Overall, prices are down 5.1 percent from the same time last year to an index gauge of 125.41 (indicating a 25 percent appreciation rate since January 2000). With unemployment still high and the economy growing slowly, more Americans are choosing to rent, hurting values. The chairman of the S&P/Case-Shiller committee described the latest news as a “double dip,” creating fears that housing could again be leading the slowing economy into recession.