Apple discovered the fraud after a number of customer complaints that their iTunes account had been compromised and that there were suspicious charges on their credit cards. While the full extent of Apple's actions aren't known, one change that has been made is you'll have to re-enter your credit card information more often.
This type of breach was bound to happen. Apple's iTunes store was a target for the same reason Windows is a favorite target of malware, it is the dominant force in its market and it has a significant user base. Even better, many iTunes accounts have credit card information stored in them. It is a target that is to juicy for the criminal element to pass up.
Keep an eye on your payment method you use for your iTunes account and change your password immediately.