Although more than 32% of American homeowners have negative equity, only about one in ten chooses to walk away from their mortgage before being forced to do so by circumstances like job loss. As long as they are able to pay, they continue to do so despite the fact that the economically rational decision would be to turn the keys over to the bank that holds the mortgage and move out.
Felix Salmon of Reuters calls this "The world's largest guilt trip." Last March, Bush's Treasury Secretary Hank Paulsen insisted that underwater homeowners "who can afford their mortgage should honor their obligations." As Salmon points out though, "he would have fired anyone at Goldman who behaved similarly."