This is from a fella who put up some Katrina evacuees. I've copied the paragraph applicable to this cash subject, the link is his full story.
Katrina

"The "bank problem." This bears consideration. I was at my bank this morning, depositing checks I'd been given by my visitors in exchange for cash. The teller warned me bluntly that it might be weeks before these checks could be credited to my account, as there was no way to clear them with their issuing banks, which were now under water and/or without communications facilities. He also told me that there had been an endless stream of folks trying to cash checks on South Louisiana banks, without success. He warned me that some of these local banks will almost certainly fail, as they don't have a single branch above water, and the customers and businesses they served are also gone - so checks drawn on them will eventually prove worthless. Even some major regional banks had run their Louisiana "hub" out of New Orleans, and now couldn't access their records. I think it might be a good idea to have a "bug-out bank account" with a national bank, so that funds should be available anywhere they have a branch, rather than keeping all one's money in a single bank (particularly a local one) or credit union. This is, of course, over and above one's "bug-out stash" of ready cash."