One of the great provisions of the Credit Card Holders Bill of Rights is that it will prevent interest rate hikes on existing balances when it becomes fully implemented by February 2010.

But the banks have found a loophole that you need to know about.

Be on the lookout for a notification about your credit card being switched from a fixed rate to a variable rate. Variable rate cards typically put the interest rate at the prime rate plus additional points.

The prime rate is low right now, which might give you a false sense of security. But as the economy recovers, it will likely rise -- even on your existing balances. And this is totally legal!

Estimates suggest that somewhere around three-fourths of all cards will become variable rate cards by February. People who run balances will most definitely be targeted.

But you don't have to take it. You can reject the notice of the change by certified mail. That will ensure there's no question that you properly notified your credit card issuer.

And if you're not running a balance and always pay in full, who cares what they do with the interest rate? The interest rate becomes irrelevant to you.

Finally, beware that some banks are now charging a monthly inactivity fee on cards that haven't been used for 12 months. That makes Clark's longstanding advice about using your "back of wallet" cards at least twice a year more important now than ever.