PlayStation Network has been down since about noon ET today. It appears to be a comprehensive outage, affecting all services.
PlayStation's official status page lists every service as experiencing issues. When one tries to login to PSN on a PlayStation 3, it shows a message saying the network is down for maintenance. None was announced as scheduled for today.
PlayStation's official Twitter support account acknowledged the outage and said it was investigating.
Update: According to Sony's network service status page — and multiple user reports — PSN services have been restored as of about 10 p.m. ET. We've had success logging into PSN and the PlayStation Store as well.