That's the problem when your group is "Anonymous", you have no central identity, no names, no affiliations. With the security of anonymity, you also open yourself to a can of worms if someone goes out and does something in your name. I'm not saying if some other guys DID pose as Anon, but if someone does choose to hack under their name, there is very little Anonymous can do to dismiss it. Simply saying "We didn't do that" is not gonna convince us.
For what it's worth, around the time PSN was getting attacked, I do remember reading articles about other websites getting attacked by Anonymous as well. Such sites taken down included Bank of America and Capital One. I just find it too coincidental that the moment Anon declares "war" on Sony, and all these other hacks occur, Anon starts saying "We didn't do it, it was someone else." Maybe it was someone else, but to expect this underground hacker community to come at large when Anon makes their declaration is a bit...hard to believe.