Update: Sony has postponed tomorrow's PlayStation maintenance until a later, unspecified date. Carry on!
The PlayStation Network will undergo "significant maintenance" on March 1 from 8AM until 10PM Pacific (that's 11AM to 1AM for those on the Atlantic; 16:00 to 06:00 in the UK).
Unlike the "regularly scheduled maintenance," the PSN will go completely dark, signing out anyone online for the duration of the maintenance.
There will be no access to the PlayStation Store, PlayStation Home, account services nor play online. The PlayStation Twitter is expected to give the clear when the maintenance is complete.