The PlayStation Network is still down and, as of yesterday, Sony still didn't know why. As stated on the PlayStation Blog :

"While we are investigating the cause of the Network outage, we wanted to alert you that it may be a full day or two before we’re able to get the service completely back up and running. Thank you very much for your patience while we work to resolve this matter. Please stay tuned to this space for more details, and we’ll update you again as soon as we can."

No other details have yet been released.