Along with a handful of other companies, it applied to the Federal Reserve late last year to transform itself into a bank holding company, a special regulatory designation for companies focused on making their money from banking activities. AmEx already ran two banking subsidiaries offering services to their customers, so it wasn't exactly a stretch.

From the companies' perspective, becoming a bank has a few big advantages and a handful of significant drawbacks. The biggest benefit, especially these days: A company must be a bank holding company to receive money from the government's bailout fund. American Express received about $3.4 billion in capital from the Treasury Department last month. Getting bank holding company status also removes some regulatory hurdles to taking deposits from its customers, which is a cheaper way to fund itself than borrowing in the bond market.

Now for the drawbacks: Banks are tightly regulated. They can't borrow as much as other kinds of financial institutions, so theoretically they won't make as much profit when times are good. And government money often comes with strings attached -- like stress tests designed to make sure its recipients won't fail if the economy keeps declining.

It will be particularly interesting to watch how GMAC performs in its stress test. Buffetted by souring auto and home loans, it had to extend the deadline on a debt-for-equity swap numerous times in order to raise enough money to satisfy regulators that it was sufficiently financially healthy to become a bank. When it ultimately closed the deal, the Treasury Department invested $5 billion from the bailout fund in the company. The stress test may give a clearer picture of whether it was enough.