It is mathematically true and it is possible but not probable at least not yet.
I believe the reason is that they keep changing the rules as they have been throughout this so called recession. An open example of a rule change was the bank bailouts.
What enables the rule changes is that there has been no formal budget set by congress since 2010, accounting tricks, and the Federal Reserve.
Geithner, his cronies, and congress are running out of rule change ideas so I expect default to occur sometime in 2012. That is if Europe doesn't collapse in the meantime. But I think the powers that be will prevent that.
The debt limit is a farce and a lie. When Medicare, Social Security, etc., is calculated into the equation with no accounting tricks the debt is really over $110 trillion.
Neither democrat or republican have a desire to solve this problem yet they created it.