The problem with saying Anonymous is responsible is that Anonymous both cannot be held responsible and, in a way, must always be held responsible.
Anonymous is not an organized group. They're a bunch of people who generally troll 4chan and related sites and react to things in varied ways. They all tend to see themselves as a collective whole, but they really aren't. If a number of people who consider themselves to be a part of Anonymous decide to work together to bring something down, then we say that Anonymous was responsible for the attack, but the fact is, there may be just as many people who ALSO consider themselves to be Anonymous who disagree with what that other part of the group did.
The problem is, ANYBODY can say they're a part of Anonymous and there's really no way to know if it's true or not. That's kind of the whole point. We don't know how many people are in Anonymous or if the people who do these sorts of attacks and claim to be a part of Anonymous ever actually interact with other people who claim to be a part of Anonymous. They have no way to build consensus or elect leaders and they generally reject this idea. They are just a mass of people who follow an ideal. That's partially what makes them so threatening.
So on the one hand, yes, the people who did this probably consider themselves to be a part of Anonymous, and so you could say that Anonymous was responsible, but on the other hand, this doesn't really feel like a typical Anonymous takedown. If the bulk of Anonymous were behind this attack, there wouldn't be any doubt in our minds of whether or not they were.
Take the recent Gawker attack as an example. When that attack occurred, Anonymous was buzzing about it and all of the stolen information was released for public consumption. In this case, while Sony says that the information has been compromised, if that much info was truly stolen by Anonymous, they probably would have done something similar, either with coordinated attacks or a mass dissemination of information. As far as I know, this hasn't happened yet. So, to me, this either seems like a small group of Anonymous "members" who found a security exploit, took advantage of it, maybe stole a bit of information for their own personal use, and then left a note as a sort of jab at them, or an attack by a different group that used the Anonymous slogan as a way of deflecting the blame.