In Quantitative Easing (QE) I, the Fed printed hundreds of billions of meaningless dollars and used them to buy government debt. In QE II, the Fed printed hundreds of billions more and bought still more government debt. Now that S&P has downgraded the US for the first time in history*, expect QE III.
QE III was on its way anyway because the insane terrorist extremist Jihadi Tea Party debt reduction deal that was just reached didn't actually reduce spending, it just slowed down the increase. Imagine the adjectives that would have been used had they actually cut the budget! I digress.
We need to borrow gobs and gobs and gobs of cash to finance our ongoing bender and if it was going to be hard to find it in Europe or Japan before the downgrade, it's going to be even harder now. Add to that the fact that with a $15T debt, even the tiniest uptick in interest rates on our bonds results in massive additional debt servicing costs and you've got a distinct need for QE III.
The only place to find the money we want to borrow and the only way to keep interest rates low is to run the printing presses at top speed.
* - Barack Obama is truly a historic president.