The move will give AmEx the ability to grow its deposits - a more stable form of funding - and provide it greater access to Fed funding and government rescue programs.
As the article mentions, AmEx already operates a bank, American Express Bank, FSB. According to FDIC data, it has $7.2 billion in deposits.
Perhaps we'll see more deposit deals from AmEx. For the last 3 years, they've been offering One from American Express. This is a spend and save program similar to Bank of America's Keep The Change program. The American Express One credit card pays 1% cash back on purchases, and the cash goes into a savings account held by the American Express Bank, FSB. It currently pays 2.75%. This is an online savings account with no minimum balance requirements or monthly fees. It appears to have an online ACH transfer service that allows you to make withdrawals and deposits to and from your linked bank account. There may also be a $50 bonus after your first purchase with the credit card (Before signing up, make sure to confirm the bonus with a AmEx rep first). One downside to this program is that there's a $35 annual fee after the first year. Perhaps with AmEx wanting to grow deposits, we'll see improvements to this program. A higher savings account yield and the elimination of the annual fee would help.
Update 11/12/08: Here's another article that a reader mentioned with some interesting commentary on this issue: NY Times: A Deal Taxpayers May Live to Regret.