The PlayStation Network is getting its oil checked and tires rotated tomorrow, September 21, and will knock several features offline during that time. The regular maintenance is expected to run from 8AM-5PM Pacific (11AM-8PM Eastern; 4PM-2AM BST) and will prevent access to the PSN Store, Home, Account Management. Users "may encounter difficulty while signing in to PSN."
Due to the maintenance, Sony will have its regular PSN update today. Follow the PlayStation Twitter for updates tomorrow on when the network will return to normal operation.