It's parked in 3 month Treasuries, whose interest rate got bid down to, well... zero... in the last auction.
So let me get this straight. The U.S. Treasury gives cash to banks. The U.S. Treasury issues Treasury Bonds to replenish its cash supply. Banks give cash right back to U.S. Treasury in exchange for 3-month Treasuries. And this cash all zooming around in a big circle is all supposed to help the economy... how?
-- Badtux the Circling-the-drain Penguin