Best short explanation of the mark-to-markets issue I have yet seen:
Imagine if you had a $200,000 mortgage on a $300,000 house that you planned on living in for 20 years. But a neighbor, because of very special circumstances had to sell his house for $150,000. Then, imagine if your banker said you had to mark to this “new market” and give the bank $80,000 in cash immediately (so that you would have 20% down), or lose your home. Would this reflect reality? Not at all. Would this create chaos? Absolutely.
More on this here (hat-tip Ramesh Ponnuru).