I have to say, I would not be surprised if Sony framed Anonymous for it. Anonymous is a super easy target and Anonymous itself likely has no way of even knowing if the accusation is correct or not. Not only that, many people will want to believe it's Anonymous. Of course, it is also possible that someone else is framing them for it.
My guess is that they were hacked by someone looking to make money from the information and they were working on getting more credit card info when Sony shut down the network. It was a good time to do it because they had been threatened by the now infamous Anonymous. The Anonymous file could have been left by them, again because the US government would like to use that to justify more raids on people's computers and would bite on that bait. Sony could also have put it there as a way to go from negligent company to victim of super-hackers. With this event potentially costing them tens of millions, if not hundreds, over the course of the next few years, they will take any PR advantage they can get.
Thank god for the disable signatures option.
