With the Fed funds rate at essentially zero, banks are borrowing money at little or no cost and using the proceeds to load up on longer-dated Treasury bonds at a substantial yield pick-up, especially when considering the leverage banks can utilize at which to do this. In other words, with a zero Fed funds rate, banks are literally printing their own cash flow by taking free money and earning a "carry" income by borrowing for free and clipping Treasury bond coupons. Isn't this the definition of a printing press?
Why do the banks require this source of liquidity if they are posting huge income every quarter now? Simple. Most of that income is derived from the non-cash source of marking up their toxic, untradeable securities like mortgage-backed securities and off-balance-sheet securities like CDOs and OTC derivatives. The banks are recognizing large "income" gains, but this does not generate cash flow.
However, if Bernanke sets the Fed funds at level at 0-.25 (current posted Fed funds is .25), banks can borrow at a 10:1 leverage ratio at .25 and purchase last week's 30yr. Treasury bond at 3.9% and generate essentially free cash flow. What a fabulous way to inject cash into the banking system in what can be viewed as a covert QE operation.
Just to put some numbers behind my theory, take a look at the two charts below. The charts show the amount of Treasury bonds/notes/bills held by banks and the Fed funds rate. Note the acceleration in bank holdings of Treausries that correlates with Ben "Buzz Lighyear" Bernanke's move to take Fed fund to essentially zero.