Still, the options for short-term investments appear more limited than before. Pan advises against moving beyond securities of the U.S. government or government-sponsored entities. He notes that other regions of the world, such as Europe, are dealing with their own financial crises.

At the same time, at least one bank, . According to this report, it will levy a fee of 13 basis points on deposits of more than $50 million. Should other banks follow suit, short-term yields could turn negative, Pan says. "Companies may have to pay for safety."

That may prompt investors to move to money market funds, which in the past few months have seen a net exodus. Institutional investments in money market funds dropped from $1.8 trillion to $1.6 trillion between early June and early August, show. Pan does not see a risk of a ratings drop for money market funds, noting that S&P affirmed its A-1+ short-term rating for the U.S.

Investment guidelines provide that minimum rating usually is required of two of the three rating main agencies, Pan says. And just after S&P announced its downgrade, on Monday, Prior to S&P's actions, it was keeping its AAA rating on the U.S., although it added that may post a negative outlook on the U.S. when it concludes its review at the end of August, according to this article in Reuters. (Perhaps further confusing things, however, is that S&P did place a negative outlook on the U.S., noting that it could drop the country's long-term rating further if the government's debt trajectory moves higher than S&P assumed in its base case.)