What Does Moody's Mean If It Downgrades U.S. Debt?

The NYT tells us that Moody's, the bond-rating agency that thought all those subprime mortgage backed securities were Aaa, is threatening to downgrade U.S. government debt if Congress doesn't meet its conditions. While the markets will probably ignore a downgrade from Moody's, just as they did the downgrade from Standard and Poor's last year (the price of U.S. Treasury bonds soared in the period immediately following the downgrade), it still would be worth asking what Moody's might mean by a...