I got message Flash, something like "Your password is no longer valid, please change it", of course the server timed out and wouldn't let me change it lol. On the bright side it might be back up, I just tried and successfully logged in (no password message this time).
As to the cause, I almost wouldn't be surprised if it wasn't a combination of things that just overloaded the network. New / free DLC (there was a PSN Plus update I think), streaming media, the BF3 Premium release and other online activity as well as whatever normal (or increased amount of) hacking attempts that just all contributed to overload the network.
I be surprised if Sony actually told us the whole story, though I am curious to know what really happened.