Sony hit another wall. A hole in the password changing procedure was found.
Sony is not out of the security hole yet. The delayed re-launch of PSN was to make sure that everything is secure as much as possible. Now an exploit surfaced on the password reset page. As reported you have to reset your password to get back into PlayStation Network.
The exploit allows anyone with access to your user account email and date of birth to change your password. Apparently this what has happened to some account according to Eurogamer.net. The exploit was first published on Nyleveia.
Sony has already reacted and the page for resetting your password is down. I also just tried to log in on Qriocity, but the login page is under maintenance. You still can use your PS3 to log in to PSN though.