It appears that Sony has taken PSN offline as of now at around 7pm GMT for maintenance. Most likely this is work to increase the security of the network in the effort to stop folks using PS3 consoles with custom firmware logging on. We'll see what happens next when it comes back up again soon.
EDIT: It seems it's online at the moment now....
EDIT 2: As of 11pm GMT it's now back up. Actually it seemed it was up before, perhaps it was certain sections of PSN that went down, I don't know.