"Now, you can believe Sony's PR team which has kept you completely in the dark, or you can see the list of events above and come to your own conclusion."
According to the quote above by poster on Reddit named chesh420, who seems to be a moderator at PSX-Scene.com, the reason for the now five-day shutdown of the Playstation Network is due to an exploit of custom firmware known as Rebug. It essentially gives users the ability to turn a retail console into more or less a dev or debug console (like the ones we have to play PS3 titles before they are released).
Apparently, modders found out that Rebug can be used to allow modded PS3s to hook up to the PlayStation Network through a proxy server and some URL manipulation, as well as send fake credit card information to the network since Sony inherently trusts the dev-specific PSN to be safe. Of course, what happened then is rampant piracy, which Sony always freaks out about and has thus begun to rebuild all of its servers to prevent this runaround.
chesh420 bases this speculation on the fact that Rebug was released March 31, that hacking guides were posted on April 7, that PSN went down April 20, and that a recent patch for the dev PS3s has been released.
Notorious group Anonymous claims that they are not behind this attack.