This article claims that about 10.3% of US homeowners (about 8.8 million) have homes that are worth less than the value of the mortgage. That's double the number of a year ago.
If the number of homes under-water can rise so rapidly with the slight decline we've seen in prices so far, I wonder what will happen when price depreciation really kicks in. Any guesses on what percentage of under-water home-owners will walk away?