Bloomberg reports one-month treasury bill rates went negative today:

One-month bill rates turned negative today for the first time since Dec. 26 as investors sought the debt approaching the end of the quarter. At that time, banks prefer to carry securities on their balance sheets instead of cash, driving demand for bills, according to Donald Galante, chief investment officer and senior vice president of fixed income at MF Global Ltd. in New York. He expects rates to rise again by mid-April.

“We’re in a funds rate range of between zero and 0.25 percent,” said David Glocke, who manages $65 billion of Treasuries at Vanguard Group Inc. in Valley Forge, Pennsylvania. “If you keep rates this low, you’re going to end up having periods, especially in the Treasury bill market, where the yield goes negative.”

The rate on the one-month bill touched negative 0.0152 percent in New York, compared with 0.03 percent yesterday. It was last negative on Dec. 26, when it reached minus 0.05 percent.

We are not so sure that the implications of another few days of negative treasury yields are so benign.  As the article says, the last time the one-month yield went negative was Dec.26.  Take a to see what the S&P did then: