I found the graph below quite astonishing.
I'm not a money market guy, but reasons I have heard for treasury bills' precipitous drop include:
The Fed calling off a $5 billion auction, leading to less supply and higher prices and thus lower yields Banks investing excess reserves in cash rather than in the fed funds market Concerns about the commercial paper and money markets, pushing risk-averse investors into the relative safety of Treasury paper
The first reason is stimulative, the second and third reasons are evidence of risk avoidance. Equities today and last Friday are viewing the move as stimulative. I think ultimately, the stock market will change its opinion to one of risk avoidance.