Ars Technica has managed to obtain a document written by the hackers, clarifiing what they did and what privacy and security risks they believe the PlayStation 3 Network outage poses.
According to the document the PS3′s connection to PSN is protected by SSL, the same system used to verify and transfer sensitive information in online purchases. This system is used by all major retailers.
The concern raised by the hackers is that custom firmwares could subvert this system as it could include custom certificates in its trusted list, allowing hackers to take your card details and use them illegally. The risks of this actually happening in reality however are not substantial as there is no generalized ability for hackers to grab credit cards from PSN users. It seems that only those using specially devised custom firmwares would be at risk.
There is no official line from Sony on this, but any news that your credit card details are safe is good news.