So the US, like every other nation on this planet (barring a few, but only a few exceptions), has zero hope of ever paying back it's debt. So it won't.
That doesn't mean the illusion can't last a few more decades or even a century or two (though only a few states managed to make it last even a single century, the most famous being the Roman republic).
The reason that everybody's happy with this state of affairs is that it allows, at this moment, an economy that's factors larger than it would otherwise be.
This situation came to be as an "accident" (that has happened hundreds of times before in history), and there simply is no conceivable way out. Anybody who can rattle the boat risks losing big, even if it's monetary interests are protected, so the situation keeps existing until someone really, really goes too far.
Apres nous, le deluge. (google this sentence)
The interest on the national debt is $260 billion per year, on nearly $17 trillion in debt, for an effective interest rate of 1.5%. If that interest rate were to rise to 1980 levels, for instance, at 6% [2] that would be an annual interest payment of over $1 trillion/year.
In order to come up with an extra $700 million per year, the government would have to raise taxes by 25%. Possibly more depending on economic factors.
Now that I have run the numbers it seems that this is not as much of a Ponzi scheme as I thought but it still seems pretty bleak. Has anyone seen this video[3] of Japan's debt problem?
[1]: http://www.usdebtclock.org/
[2]: http://www.usgovernmentspending.com/year1980_0.html - It appears that the government spent on interest about 6% worth of its total debt.
In any case, a worrying level of debt does not make something a Ponzi scheme...
The crucial difference between what the U.S. is doing and a Ponzi scheme is that the U.S. is actually generating income, to the tune of 1/4 of the world's economy. That income stream is missing in a Ponzi scheme. Paying off maturing debt by refinancing it with new debt is nothing unusual--corporations do it all the time. It's fine because there is the underlying income stream.
Now, the fact that it's not running a Ponzi scheme doesn't mean the U.S. can continue borrowing forever. Creditors might get nervous if the debt levels get too big relative to the income stream. What the U.S. has to worry about is not how much debt it's taking out now. What the U.S. has to worry about is some other country becoming a more attractive place for the world to stash its cash.