According to the PlayStation US Blog: “An external intrusion on our system has affected our PlayStation Network and Qriocity services. In order to conduct a thorough investigation and to verify the smooth and secure operation of our network services going forward, we turned off PlayStation Network & Qriocity services on the evening of Wednesday, April 20th. Providing quality entertainment services to our customers and partners is our utmost priority. We are doing all we can to resolve this situation quickly, and we once again thank you for your patience. We will continue to update you promptly as we have additional information to share.”
Anonymous states they have nothing to do with PSN going down, which means one of two things, if Anonymous is telling the truth: another hacker group brought down PSN, or Sony screwed up the servers themselves, perhaps trying to add security measures. Who knows what the truth is here? Blaming hackers would be a convenient out for Sony PR-wise with the network being down for the last few days.