In a post on the Official PlayStation Blog tonight, the Chief Information Security Officer for Sony Phillip Reitinger informed users that a major, concerted wave of sign-in attempts were made using login and password information "obtained from one or more compromised lists from other companies, sites or other sources." Sony has, in their words, "taken steps to mitigate this activity." Reitenger specified that credit card information had not been compromised.
Accounts affected by the mass access attempt have received prompts via email to reset their passwords. Sony Online Entertainment accounts affected have also been provisionally disabled until their passwords are changed.