Thanks for the clarification. Your question is "What would happen to unemployment if all the people in the US whose houses are underwater were to simply walk away?"
It's a great question. So I sent it to Professor Steve Keen in Australia. And I'm going to ask Cullen Roche, who writes the wonderful blog pragcap.com
Mr. Keen is on Facebook, so I'll let you know if he writes back to me. And I'll tag you when I ask Cullen Roche in G+.