Last year, the state of Maryland had more than $200 million on deposit in a Citigroup account that was fully insured by the Federal Deposit Insurance Corp. and that paid 50 basis points. "It was an extremely wise investment," says Mary Christine Jackman, the state's director of investments. Then Citi, like most big banks, opted out of the unlimited FDIC coverage. Jackman immediately pulled the money and invested it in things like Treasuries, agencies and repos.

"Without the insurance, it was no longer attractive," she says. "The whole point was getting the insurance." Making sure investments are insured or guaranteed is one reason Maryland has a triple-A rating, she points out.

Beginning in October 2008, the FDIC's transaction account guarantee, or TAG, program, insured the full amount in accounts, rather than just the $250,000 normally covered. Last year, TAG was extended through this June, but more than 3,100 banks have opted out, including Bank of America, Bank of New York Mellon, Citigroup, HSBC, JPMorgan Chase, PNC, SunTrust, USBank and Wells Fargo. Comerica and KeyBank had not opted out at press time.