Not sure if this is already being discussed on the forums yet. If so, close topic.
As it turns out, the reason for the PSN outage was a hack that would let you go online with older firmware. in addition you could connect to the developer server which doesn't validate your credit card when purchasing items from the Playstation Store. Obviously once this was found out and leaked on to the various PSN scene and hacking forums, people probably started downloading as much as they could and as fast as they could.
I'd just like to point out how fucking insane this is.
This is a really interesting post from some dude on Reddit who claims to be a mod on PSX-Scene: I'm a moderator over at PSX-Scene.com - The real reason PSN is down.
Ok, I've seen a bunch of speculation of why people think PSN is down, and I thought I should just post what the community knows in comparison to what Sony is telling everyone. The truth is, there was a new CFW (custom firmware) released known as Rebug ( http://rebug.me). It essentially turns a retail console into a dev console (not fully, but gives you a lot of the same options that usually dev's only have access to). Anyway, this new CFW was quickly figured out by 3rd parties (not Rebug) to give CFW users access to the PSN network again via the dev networks. With a little manipulation of the URL's through a proxy server you could get your hacked console back online. Not that big of a deal, right? Well, it also turns out that some people over at NGU found out that you could provide fake CC# info and the authenticity of the information was never checked as you were on Sony's private developer PSN network (essentially a network that Sony trusted). What happened next was extreme piracy of PSN content. Sony realizing the issue here shut down the network. Now, before you go freaking out about the latest information posted about Kotaku, no ones personal information was accessible via this hack. Not to say they couldn't get it, but no one is admitting to it being available. Anyway, that's the real reason for the PSN downtime. Sony is now rebuilding all of it's PSN servers to be more secure and (hopefully) make sure the CFW users cannot get online anymore.
Edit #1: To those of you saying that this is speculation, you are correct. But, it is speculation based on a lot of facts and the outcome seems to make the most sense.
Rebug was released on 3/31/11.
First guides of how to use the dev network to get back on COD games on 4/3/11.
Word of NGU users finding a way to pirate PSN content via the dev networks on 4/7/11 (basing this on posts I had to delete on the website. Update: Users have pointed out to me that these posts existed on NGU as of 4/2/11).
PSN goes down on 4/20/11
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. Now, this isn't the first time Sony has fought back against the PS3 modders from getting on PSN. A couple of months ago we had a utility called f*ckPSN that changed the necessary header information that was being sent to Sony to allow modified consoles back online. We were able to use it for about a month. Then came the new TOS, the mass e-mail to PS3 customers, and software update 3.56 and 3.60. So, once again, yes this is all speculation, but it is speculation based on previous actions and known facts.
Edit #2: Mathieulh just mentioned that he has been in contact with someone that has official access to the SCE devnet servers and it was posted to them today that only 3.60+ debug firmwares will be allowed on the dev network anymore. All earlier versions will be cut. If you want to retain your access you need to contact Sony and upgrade to 3.60 debug firmware.
Edit #3: Ok, it looks like some various news sites have picked up this story and taken it out of context. Once again, this is all speculation and information gathered from various devs in the PS3 scene. It might very well not be the real reason PSN is down, but as the timeline fits, it's a reasonable explanation. Now, as to Rebug directly allowing this to happen, that's not the case at all. Different CFW's have had access to the dev network the whole time. This is not new news for people in the PS3 scene. It's what people have figured out what to do with the said network that has caused all the recent issues. Saying that Rebug is what did this is like saying a gun manufacturer is responsible for every death that happens with a gun.
Edit #4: Looks like Sony is finally admitting that people have been able to get into their network and users personal info has possibly be compromised. See: Official Sony Blog for the latest update.