The national debt, as it is commonly reported, is actually the "debt held by the public," which was $4,419,348,484,874.18 on 12/16/2004. But the "General Fund" of the federal government, which is the accounting unit responsible for the national debt, actually owes much more. It owed $7,519,646,830,227.83, also on 12/16/2004. The extra $3+ trillion is owed to the Social Security Trust fund and several other government trust funds. The smaller number is reported because politicians like to hide the larger one, and it is justified by economists who point out that it is borrowing from the public that affects the economy as a whole.
While the economists are right, this misses the political implications of pretending that money owed to Social Security doesn't count. If that money is all spent as under Presidents Reagan, W. H. Bush, and W. Bush, then it will be hard to pay back, while if it is used to pay down the Gross National Debt, as under Clinton, then it can easily be paid back by re-borrowing from the public. (National Debt Graph showing presidents.)