Buried toward the bottom of a gloomy report on home foreclosures is this:

The threat of so-called “mortgage walkers,” or homeowners who can afford their payments but decide not to pay, also increases as home values depreciate and equity diminishes. Banks and credit-rating agencies already are seeing early evidence of this.

Banks should persue these folks aggressively!
Really, someone thinking of moving that is in this situation probably shouldn’t be moving unless their new employer has a great relocation package. Their creditors should make sure that the potential perps know that they will have to pay out lapsed payments and any shortfall in equity.
There were quite a few folks in this situation in the early to mid ’90s. During that time I was in a negative equity position for a number of years but time cures many ills and even in the current market I am substantially positive.

During that same period I spent a brief stint as a realtor and sold a neighbors house. As part of the transaction his corporate relocation folks had to pick up nearly $100,000 in negative equity. Those times were not pleasant and a lot of people simply had to stay put until market values turned up.