Sony has announced that PlayStation Network will undergo routine maintenance today, monday. During the downtime, Sony is going to work on deploying a series of back-end improvements to the network.
The maintenance is scheduled to take place from 5:00 PM GMT (9:00 AM PDT) to 11:00 PM GMT (3:00 PM PDT). Like usual, players that sign in before the maintenance starts will be able to continue playing online, but won't be able to access PlayStation Store, PlayStation Home, and Account Management.
Lastly, Sony adds that some additional downtime might be required after the maintenance period for testing purposes.
Source: Playstation Blog