From the WSJ:

In bailing out AIG, the Federal Reserve appeared to be motivated in part by worries that Wall Street's financial crisis could begin to spill over into seemingly safe investments held by small investors, such as money-market funds that invest in AIG debt.

Indeed, on Tuesday the $62 billion Primary Fund from the Reserve, a New York money-market firm, said it "broke the buck" -- that is, its net asset value fell below the $1-a-share level that funds like this must maintain. Breaking the buck is an extremely rare occurrence. The fund was pinched by investments in bonds issued by now collapsing Lehman Brothers.

Money-market funds are supposed to be among the safest investments available. No fund in the $3.6 trillion money-market industry has lost money since 1994, when Orange County, Calif., went bankrupt. A number of money-market funds own securities issued by AIG. The firm is also a big insurer of some money-market instruments.