You're cut off – Reason Online poses this question in "Debt Wish": "In other words, despite some anxiety, investors don’t seem too worried about U.S. debt today. But at the point when investors become more worried, might it be too late?" A corollary question is "When credit is cut off, will we have the power to pay off the debt?" It's a question that Greece is – painfully – trying to answer right now.
Related - Atlantic: "5 Doomsday scenarios for the U.S. economy."