Heads up folks! Sony have announced that the PSN will be offline this evening from 17:00 - 19:00 BST due to ongoing maintenance. The maintenance itself will actually run from 17:00 through to 21:00.
As usual, you won't be able to access the PlayStation Store, account registration or account management, and you won't be able to sign in during the downtime. If you sign in beforehand, though, you should be fine.
Playstation Home gets a massive v1.55 update today, which may well have something to do with it.