The United States will not default because it pays its debts in its own currency, (and the Treasury can always just print more money) but the prospect of a ratings downgrade is quite real. That means it would cost considerably more to finance the debt. Also, long-term interest rates will rise sharply. That will crimp consumer spending, slow economic activity, and deal a death blow to the struggling housing market.
Bernanke's playing a dangerous game. If he's not careful, he could trigger a run on the dollar.