Top execs at big U.S. banks have said that a debt default by the United States is unthinkable and probably won't happen. But many financial institutions have admitted that they're still engaged in debt disaster planning. Citigroup (C, Fortune 500) CFO John Gerspach said Tuesday morning that the bank "remains hopeful" a deal can be worked out to avoid a default. But he added that "hope is not a plan" and that the bank has prepared for different contingencies over the past few weeks. So what could happen in a worst-case scenario if the U.S. actually defaults? It's impossible to predict. But here are six ways that ... (full story)