The housing boom is acknowledged largely because housing was accumulated at a faster rate 2002-2006 than it was in 1990s.
Maybe less recognized is that real per capita incomes grew 0.8% less per year 2000-2008 than they did in the 1990s. Population grew 0.3% per year less. Based on these factors alone, housing demand needs to increase 7% more in order for housing prices to stop falling. This could take a while.
However, I believe that there were at least some fundamentals other than income and population driving the housing boom, and those fundamentals persist today. Especially important is information technology, and its effects on the banking and real estate industries. Thus, from a theoretical perspective, it is possible that housing prices have already finished falling.
Interestingly, the OFHEO (now called the FHHA) released its price index for February. It shows that housing prices increased Jan-Feb, after an increase Dec-Jan. According to OFHEO, housing prices even increased in the Pacific and Mountain regions.
As noted by several commenters, the OFHEO index and the Case-Shiller index do not always agree. Nevertheless, it becomes more and more difficult to argue that housing prices and construction are falling in 2009 the way they were in 2008, and easier to argue that the housing market has hit bottom.