Not the first time it is claimed this has happened but this time its official with warnings being sent out to PSN users by Sony themselves:

Notice to PLAYSTATION®Network Users

We have found out that there has been a possibility of unauthorized access to personal information on the PLAYSTATION®Store through PCs, a content download service of the PLAYSTATION®Network. Although unlikely, it is possible that the passwords of a small percentage of PLAYSTATION®Network users may have been changed through unauthorized access, making it possible to view users’ personal information and/or use the Wallet for the PLAYSTATION®Store. PLAYSTATION Network accounts do not display entire credit card numbers, so any unauthorized access to your PLAYSTATION Network account is very unlikely to compromise your credit card number.

We have taken immediate measures to rectify this issue and system security is restored.

We have investigated the extent of unauthorized access and possible alteration of passwords that could have occurred before corrective measures were taken, and are directly contacting customers who may have been affected by this incident. In order to verify that your account is intact, we strongly suggest that PLAYSTATION®Network users sign in to the service. If you can successfully sign in with your pre-set password, your account is not affected by this incident.

For more information, please contact Customer Service.

Remember; do not disclose your log-in or password details on any email communications.

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