The S&P/Case–Shiller U.S. National Home Price Index is a composite of single-family home price indices for the nine U.S. Census divisions. It is updated monthly. The national index is normalized to have a value of 100 in the first quarter of 2000. (Wikipedia)
What is a double-dip?
As a former trader we had a saying that nothing goes straight down. A severe move down will often be followed by a bounce in price before resuming the downward move. This is what is called a double-dip. This move happening is not 100% given, but it happens more often than not.
A fear is that the bottom that was set for home prices in Q1 2009 may not hold creating further angst for an already weak economy trying to find firm footing.