A moderator at PSX-Scene, Chesh, has posted an educated guess on Reddit as to why Playstation Network is down—and has been for the past six days—but his explanation does not involve the Internet group Anonymous (who, in any case, have denied responsibility). Sony has officially blamed an “external intrusion”, and unplugged PSN to investigate and rebuild the system.
Sony was originally targeted by Anonymous after SCEA sued Geohot for jailbreaking the PS3. Anonymous posted demands earlier this month, but made it clear that they would steer clear of PSN, as they “are not after the players“.
Chesh believes that a custom firmware for the PS3, “Rebug”, is the source of the issue. Rebug essentially transforms a standard retail PS3 into a developer console, allowing it to perform secret options normally reserved for proper devs. Users (not Rebug directly) modified the firmware to revive hacked consoles and pirate PSN content. Because of this intrusion, Sony decided to shutdown PSN. As far as Sony knows, no personal information, such as credit card details, have been compromised, but they are still researching the extent of the damage.
Read on for more details, but keep in mind that this is still complete speculation.