It's Official: External Intrusion Is Behind PSN Outage
Posted on April 23, 2011 by Jason Leyanna 
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.
Source:
Official PlayStation Blog