Sony has said that certain PlayStation Network services will experience downtime today between 4.05pm and 1am tomorrow morning. (Downtime? What's that?!)
According to the official PlayStation Blog, said downtime will render the PlayStation Store, PSN Account Management and Account Registration inaccessible.
However, if you happen to sign in before 4.05pm GMT today you should be able to play online anyway, and whether or not you're signed in you will still be able to collect in-game Trophies, which will be synched with your profile once PSN is up again.
It's all part of scheduled maintenance, apparently, so don't expect anything too special to have changed when the service pops back up. Probably.