From the Playstation Blog:
"PlayStation Network will undergo a significant maintenance period on Thursday, March 1st. Maintenance is expected to start at just after 8:00 am Pacific and will last for several hours, until approximately 10pm Pacific. Unlike typical maintenance events, consumers who are already signed-in to PSN will be signed-out, and consumers will be unable to sign in for the duration of the maintenance window. During this time, users won’t be able to access the PlayStation Store, PlayStation Home, Account Management or play online."