If there is a problem, the Fed will drop interest rates, maybe even go negative, and that will cause bond rates and conceivably mortgage rates to drop as well.
The Fed wants inflation, and most importantly home price inflation. They will do whatever it takes to stop deflation, they've already said this. Bernanke said he would drop bags of money out of helicopters, obviously an exaggeration, but basically this is how critical the Fed views the fight against deflation.