From Calculated Risk:
...By the end of 2007, prices had fallen 10% from the peak, and 8.2 million homeowners owed more on their mortgages than their homes were worth.
As of Q2 2008, prices had fallen almost 18% from the peak, and for the graph, I estimated that prices will decline about 22.5% from the peak by the end of 2008. (this seems conservative). This means about 15.4 million households will be underwater or already foreclosed on by the end of 2008.
The last two categories are based on various estimates for the price bottom (peak-to-trough). The 30% decline was suggested by Paul Krugman in December 2007: What it takes). The 35% decline is close to the "severe recession" case presented by JPMorgan last week....MORE