The @PlayStation account tweeted late last night, "Though estimated to end at 8pm tonight, maintenance has been extended to tomorrow AM. Updates as we get them."
This morning, the @PlayStationEU account announced, "Unfortunately scheduled PSN maintenance has over-run. The service is not expected to return until 18.30GMT. Apologies for the inconvenience."
Update: The completion of PSN maintenance in Europe is now delayed until 10PM GMT. As @PlayStationEU tweets, "Apologies for the inconvenience, but we are now not expecting PSN maintenance to conclude until around 10.00pm GMT."
Update 2: A PlayStation Knowledge Center update says PSN maintenance in North America is now scheduled to end tonight at around 9PM PT.
Update 3: So much for that. PSN downtime in Europe is now extended through to Saturday morning. @PlayStationEU tweets: "Unfortunately PSN maintenance is still underway and service is now not expected to resume before tomorrow morning. We'll update again soon."