Sony has announced that the PlayStation Network (Sony Entertainment Network) will be undergoing "significant maintenance" on the 1st of March, from 8AM until 10PM PST.
Unlike previous scheduled maintenance, anyone who is signed in will be locked out during the duration of the maintenance. This means that there will be no access to the PlayStation Store, PlayStation Home, accounts services and online game play throughout the time period mentioned above.
The official PlayStation Twitter account is expected to notify users when this maintenance will be completed, or if there are any complications during the maintenance period.
Source: PS Blog US