Sony will take PlayStation Network down this evening from 4pm UK time to half-past midnight - a entire evening's worth.
You can play online during that time if you log-in before 4pm, a post on the European PlayStation blog explained. Trophies can be earned while gaming offline as usual, and they'll update when PSN comes back online.
You won't, however, have access to the PlayStation Store on PC, PS3 and PSP, nor PlayStation Network Account Management and Registration.
Sony cites "maintenance" as the reason for the outage.