(FORTUNE Magazine) – YOU'D THINK THAT ONLY A WARREN Buffett would qualify to put $600 million on his American Express card. Yet, in effect, that's exactly what Delta Air Lines CEO Gerald Grinstein just did. Between AmEx's advancing that sum and Delta's pilots' agreeing to $1 billion in pay concessions, his wobbly company has, for now, staved off bankruptcy.

American Express came through for the carrier because the two companies are joined at the wallet by the Delta SkyMiles credit card. The airline business is in a shambles, but airline-affiliated credit cards are gold mines.

Balances typically run four times higher than on the average card, because users consolidate their charges on the cards to rack up frequent-flier points. They're also a low-risk proposition, because of the affluent customer base. "It's a really, really profitable business," says Randy Petersen, editor of InsideFlyer. So issuers have a strong motivation to rush to the patient's bedside when their golden goose is ailing.

Bank One extended $600 million in credit to United Airlines in December 2002 when its card partner went into Chapter 11. Citibank stood ready to finance partner American Airlines in April 2003, when it was near Chapter 11 (American averted bankruptcy with last-minute labor concessions). In Delta's case, American Express is extending just $100 million of credit but prepaying $500 million for SkyMiles points. (Card issuers typically pay airlines around a penny per mile for the points.) That's a critical infusion for Delta, whose cash had dropped below $1.5 billion at the end of the third quarter.

Not everyone is cheering about the little green card coming to the big airline's rescue. American Express may not be so much a standup partner as a co-dependent. "This is another example of enabling an airline to defer reality," writes Roger E. King of CreditSights. "It does not address Delta's competitive position or its pension overhang." Rather, he concludes, it's another instance in this industry of "capital infusions that delay the day of reckoning." -- John Helyar