Sony is currently performing maintenance on the PlayStation Store. The PSN scheduled downtime is from 9:00 AM PT to 2:00 PM PT.
A post on the US PlayStation support site says that some users may experience issues logging into the PlayStation Store or PlayStation Home and may receive a message stating, “Sony Entertainment Network is undergoing maintenance.”
If you have not logged into the PSN within 5 days prior to the scheduled maintenance, you might experience problems with apps such as Netflix, HULU Plus or Music Unlimited.
Sony has also acknowledged that some authenticated users may have lost access and it's working hard to restore access as soon as possible.
You can receive notice when the PSN is fully restored via Twitter from @PlayStation or @AskPlayStation.
UPDATE - PSN maintenance extended.