This strikes me as a poor security trade-off. It reduces a rather unlikely threat (you are using SSL right?) in exchange for greatly increasing the value of your database to an attacker.

Amen.

Not to mention the additional risk of a "trusted" insider getting access to the database. Most fraud is an inside job rather than the results of the wily hacker.

If you really have to store the credit card numbers then for goodness sake don't store them in plain text.

For example XOR them with the users password so they can't be accessed on mass, and only decoded on an individual basis when the user is logged on.

(I'm naturally assuming that you'll only be storing a hash of the user password.)