US Downgrade Danger?

I saw an article today in the Financial Times that Moody's was warning that the US may face a downgrade to its AAA credit rating unless it can achieve either more robust economic growth or do something to limit its increasing need to borrow. Obviously, such a downgrade would inspire political sniping and cause the government to pay more for its debt, but could there ever be a real danger of default? Since the US uses a fiat currency, in such a case where they lacked the funds to service the debt (or could not get anymore credit) they would just print new money. Clearly, this would devalue the dollar but it would also prevent an actual default. There is value in ratings for municipalities, states, corporations and countries that do not directly control their currency (the EU countries), but there is no real harm in a downgrade to a country like the US or the UK who can just print more money to cover their interest expenses. I didn't think it was Moody's (or any other agency's) responsibility to rate the projected future value of money as it appears they are doing with this morning's warning.