Rewards cards are a particular concern. People who use rewards cards tend to spend more than those who use standard cards or other payment methods, the GAO said, and retailers question whether those increased sales are worth the higher interchange fees the rewards cards command.

"Staff from (large retail chains) all expressed concerns that the increasing use of rewards cards was increasing merchants' costs without providing commensurate benefits," the report said.

A raw deal for retailers?

Retailers say they have little wiggle room to negotiate these charges because their merchant agreements with the credit card networks tie their hands. Under those agreements, they must accept all types of plastic if they accept one. (If they accept credit cards, they must accept debit, and vice versa. Plus they can't discriminate against high-cost rewards or business cards.) They can't impose surcharges for credit cards or steer customers to cheaper payment methods.

Other countries have dealt with rising interchange fees by changing the agreements the card networks have with banks and merchants. Among them:

  • Limiting or capping interchange fees.
  • Allowing merchants to impose a surcharge for accepting cards.
  • Allowing retailers to accept only debit or credit cards instead of requiring them to take both.
  • Letting merchants steer customers to lower-cost payment methods.

Congress has begun to tackle at least some of these concerns. The Dodd-Frank financial reform measure that became law in July 2010 gives the Federal Reserve increased regulatory authority over debit-card swipe fees and aims to limit anticompetitive practices in handling card transactions.

The legislation could also be the beginning of the end for truly great credit card rewards. Anything that lowers revenue from interchange fees could result in such cutbacks. From the consumer's side, using rewards cards might become harder to justify if you lose a discount that could offset the value of the rewards.

Of course, we all know there is no such thing as a free lunch, and the deals that come close don't last forever.

Here's one example. For a few years, many people used a credit card arbitrage scheme to make easy profits. They used fee-free, 0% balance-transfer offers to write themselves checks that they then deposited in savings accounts earning 3% to 5%. One message-board poster claimed to have borrowed a whopping $250,000 that way, which generated interest income of more than $1,000 a month.

Those deals are long gone. You're lucky to get much above 1% on a savings account now, and card issuers have raised fees and rates on balance-transfer offers.

The same thing may well happen with rewards credit cards. A good thing may have become too good to last. So if you have rewards cards, use those rewards while you still have them.

Liz Weston is the Web's most-read personal-finance writer. She is the author of several books, most recently "The 10 Commandments of Money: Survive and Thrive in the New Economy" (find it on Bing). Weston's award-winning columns appear every Monday and Thursday, exclusively on MSN Money. Join the conversation and send in your financial questions on Liz Weston's Facebook fan page.