Even absent such risks, low and falling inflation indicate that the economy has considerable spare capacity, implying that there is scope for monetary policy to support further gains in employment without risking economic overheating. The FOMC decided this week that, with unemployment high and inflation very low, further support to the economy is needed. With short-term interest rates already about as low as they can go, the FOMC agreed to deliver that support by purchasing additional longer-term securities, as it did in 2008 and 2009. The FOMC intends to buy an additional $600 billion of longer-term Treasury securities by mid-2011 and will continue to reinvest repayments of principal on its holdings of securities, as it has been doing since August.
It's the only arrow they have left in the quiver, and it's kind of a dull one at that.
Ben S. Bernanke - What the Fed did and why: supporting the recovery and sustaining price stability
Many outside the Fed, and some inside, see the move as a 'Hail Mary' pass by Fed Chairman Ben Bernanke. He embraced highly unconventional policies during the financial crisis to ward off a financial-system collapse. But a year and a half later, he confronts an economy hobbled by high unemployment, a gridlocked political system and the threat of a Japan-like period of deflation, or a debilitating fall in consumer prices.
Fed to Buy $600 Billion of Treasurys - WSJ.com