I'm always peeved by this reference to Keynes. This is a topic that is universally recognized by almost all economists, with the exception of a few Austrians. In fact, the main proponent of this concept was a conservative economist from the University of Chicago named Irving Fischer. Because the Great Depression was a period of absolute deflation (falling prices and falling output), this was a central concern to Keynes as well. However, it was none other than Milton Friedman, a lead figure in the resurgence of neo-liberal (libertarian) thought in the mid 20th century. In fact, Milton Friedman's most famous work (the work with Anna Schwartz), was all about how the Fed caused the double-dip in the Great depression by enforcing the gold standard and contracting the money supply. See? Friedman (a libertarian) rose to fame for his claim that the Fed destroyed too much money during the Great Depression!

Edit: One quick thing I forgot. Keynesian economics is all about the idea that, if prices are "sticky" and cannot fully adjust in real time, then that leads to drops in output. The flip side of this, which is evident in the current crisis, is downward nominal wage rigidity. The fact that prices are slow to adjust partially mitigates the threats of deflation. So if anything, Keynesians have less to worry about from deflation than other factions of macroeconomists.