US national debt ceiling

Last year, Tim Geithner declared the US would never lose its AAA credit rating:

http://www.businessweek.com/news/2010-02-08/geithner-says-u-s-will-never-lose-aaa-debt-rating-update1-.html

Treasury Secretary Timothy F. Geithner said the U.S. is in no danger of losing its Aaa debt rating even though the Obama administration has predicted a $1.6 trillion budget deficit in 2010.

“Absolutely not,” Geithner said, when asked in an ABC News interview broadcast yesterday whether a downgrade is a concern. “That will never happen to this country.”

Yet, in the last few days, Geithner has also said the US would default on its bond obligations if Congress doesn't raise the national debt ceiling.

http://www.washingtonpost.com/wp-dyn/content/article/2011/01/16/AR2011011604005.html

In a letter to Congress, Geithner said the statutory debt ceiling of $14.3 trillion, set last year, may be reached by the end of March - and hit no later than May 16. He warned that holding it hostage to skirmishes over spending could lead the country to default on its obligations, "an event that has no precedent in American history." Debt-level brinkmanship doesn't wear a party label.

So the $64 trillion question is:  how can a country with the highest credit rating be at risk of defaulting on its debt?  A country defaulting on its bonds is usually a banana republic assigned junk-bond status.  So which is it, Tim?  Does the US deserve its AAA credit rating?  If so, how can the US be at risk of default?  When will the American public be told the truth?