Yes, in the long term, printing money will lead to an inflationary spiral. The expectation of that will cause higher interest rates. But the fact remains, that in an emergency, it is a possibility. And the possibility of creates greater safety/willingness to lend.

Moody's in its latest assessment says:

"The global role of the dollar, which underpins continued demand for U.S. dollar assets, .... provides unmatched access to financing, meaning that the U.S. government can support higher debt levels than other governments"

http://economix.blogs.nytimes.com/2011/08/08/moodys-why-the-...