When Wall Street opens tomorrow, we will know how serious it regards Standard & Poor’s decision last Friday evening to downgrade the credit of the United States.

The decision, criticized by the U.S. Treasury, has dominated the political debate over the weekend, which has mainly been concerned with how the downgrade from AAA to AA+ will affect the U.S. economy’s recovery and if the risks of a new — “double-dip” — recession have increased.

Former Federal Reserve Chairman Alan Greenspan said on NBC’s “Meet The Press” today that the U.S. economy actually has been improving, albeit slowly, and that the risks of a new recession much depend on what happens in Europe, now that Italy’s large economy exhibits such weakness. But no one, including Greenspan, has questioned if the U.S. can meet its obligations and the country’s treasury bonds are still the world’s most secure investment.

Warren Buffet said that if there was such a thing as an AAAA (four) rating, he would give the U.S. that grade, according to a nice summary by The Atlantic Wire of responses to the S & P’s decision. Much of the debate concerns Standard & Poor’s tarnished reputation after so many years ov giving the highest credit ratings to the subprime loans, which led to the real estate crash. Many in the debate question the S&P’s credibility after that debacle and now question its decision.

Robert Reich writes on his blog:

“Had Standard & Poor’s done its job over the last decade, today’s budget deficit would be far smaller and the nation’s future debt wouldn’t look so menacing. We’d all be better off had S&P done the job it was supposed to do, then. We’ve paid a hefty price for its nonfeasance. A pity S&P is not even doing its job now. We’ll be paying another hefty price for its malfeasance today.”

Ezra Klein writes on his Washington Post blog that “Standard & Poor’s did not just miss the bubble. They helped cause it,” and adds, “the credit-rating agency model is broken and, at times, dangerous, and investors need to pay less attention to their pronouncements.”

So, how will Wall Street react tomorrow to S&P’s downgrade? It is uncertain, but it’s important.