More on this from here [PDF]. The bottom line is that there are two options: (1) spending is cut drastically and taxes go way up or (2) the Treasury issues bonds and the Fed prints dollars to buy them. Most likely it will be a combination of both with (2) being easier for re-election-seeking politicians to do than raising taxes or cutting spending and services.
And the scary part is that medicare has been able to control its costs as well as or better than private insurance. And...