With property values still tumbling, it's no surprise that nearly a quarter of the nation's mortgage borrowers owe more than their home is worth. Industry studies support the consensus that the farther a borrower sinks into negative equity, the more likely they are to throw in the towel. The severity of this catch-22 is now top-of-mind for government officials. The administration is reportedly pressuring Fannie Mae and Freddie Mac to make principal write-downs a key component of their foreclosure prevention efforts.