Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of credit card giants Visa (NYSE: V) and MasterCard (NYSE: MA) both dropped more than 10% today on concerns over regulation of debit card fees.

So what: As part of the Dodd-Frank financial reform bill, regulators are looking into placing a cap of $0.12 on debit card interchange fees. The interchange fees are the fees that banks (the card issuers) charge the acquirers (the folks that connect merchants to the network) when a transaction is processed. Visa and MasterCard don't actually get any of the interchange fees, so it's the major card issuers like Bank of America (NYSE: BAC), Capital One (NYSE: COF), and Citigroup (NYSE: C) that will be hit in the pockets if this cap goes through. However, this is still a concern for Visa and MasterCard since they depend on the attractiveness -- to all parties involved -- of using cards as a payment form. If government regulation suddenly makes it less attractive for banks to issue debit cards, then volume on the Visa and MasterCard networks could be affected.

Now what: It's always an adventure when Uncle Sam gets involved. Some banks have released estimates on what this could cost them. Bank of America, for instance, said it could run between $1.8 billion and $2.3 billion in annual revenue. The cost to Visa and MasterCard is far less clear, and, if banks make up for the lost interchange fees by boosting fees elsewhere, the impact could be fairly minimal. But the broader risk here is that once the government gets involved it will dig in further and cause larger disruptions in the card markets.

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Fool contributor Matt Koppenheffer does not own shares of any of the companies mentioned. You can check out what Matt is keeping an eye on by visiting his CAPS portfolio, or you can follow Matt on Twitter @KoppTheFool or on his RSS feed. The Fool's disclosure policy assures you no Wookiees were harmed in the making of this article.