The New York Times has a helpful guide for customers of the most troubled banks.

The short hand is this: whatever you have at a bank—cash, investments, mutual funds—that is, most likely, yours. If your cash holdings are less than $100,000 and it's an FDIC bank, you are really fine.

If you have a 401(k) or whatever, you own that. The company that sends you your statement is just providing a service to the mutual fund owners (which are the investors, i.e., you).

If you hold stock or bonds at some brokerage, that's yours. They're just providing you with the service of holding them for you.

In short, you almost certainly won't lose money. But there might be a period of time (hours, days, unlikely to be as long as weeks) when you can't access your money because of the turnover time. You probably will have to learn some new phone numbers and a new customer website, etc.