The Securities and Exchange Commission threw in the towel on tightening regulations on money-market funds in August, although regulators have a number of options to continue the fight. Still, money funds' role as one of the main havens for corporate cash may have passed.

Companies' use of money funds appears to be waning. Ben Campbell, president and CEO of Capital Advisors Group, says the SEC's Rule 2a-7 effectively reduces the weighted average maturity of money fund investments to 60 days, and in practice maturities are now under 50 days. That's down from an average of 90 days prior to the financial crisis and 120 days before 1991, Campbell says, and puts money fund yields at about 0.1%.

"Our conclusion is that [treasury executives] are going to consider a wide variety of investment channels outside of traditional MMFs," Campbell says.