PlayStation Network will be down for routine maintenance starting at 8:00AM PST today. The service is expected to be down until 8:00PM PST tonight.
During this downtime, the PlayStation Store, PlayStation Home, and account management stuff will not be accessible. You will be able to play online and access your apps so long as you were signed into PSN before the maintenance starts up today, however.
[MAINTENANCE] Scheduled PSN Maintenance April 15, 2013 [PlayStation Forums]