From David Goldman at Inner Workings:
An aging population increases its purchases of securities and decreases its purchases of goods as it saves for retirement. Americans have saved nothing for the past ten years, and the capital gains that they considered savings-substitutes have vanished. That means that an enormous savings deficit accumulated over more than a decade has been exposed, and that Americans must attempt to correct it quickly and under the worst of circumstances. That creates a deflationary shock that a few trillion dollars’ worth of stimulus cannot begin to mitigate. America may have the worst of both worlds: currency devaluation AND price deflation, as in the 1930s.