If there's a run on the American dollar, it doesn't matter which brokerage or bank your money is with, you'll lose value. If you hold something like gold ETF's or Asian stocks you might protect yourself in that situation.
Ken Lewis has your money nice and safe in a black bag he carries. I think Mozillo also stand behind your accounts
MF Global retail customers funds are in Harris Bank same as RCG. Institutional funds are in JP Morgan Chase. All MF Global Foreign wires (EUR, CHF, JPY) go into BAC. Euro deposits for RCG go into BAC also, GBP's for RCG go into Barclays and CHF's go into Credit Suisse. Interactive Brokers funds go into Citibank but they say they keep most of the funds in BAC, JPM, Deutsche Bank, and smaller amounts into C, HSBC, USBank . Thinkorswim funds go into JPM. I think if you keep dollars, GBP or CHF in RCG you have zero exposure to BAC. I'd be more worried if RCG kept any funds in Citibank.