There was an initial round of finger-pointing towards North Korea, and now a bunch of people saying, hold up, this doesn't really make sense for North Korea to be behind the attacks. OK, it's not logical, but as as the previous poster argues, 1) North Korea isn't logical (or rather, they are logical but employ something rather different than the logic found outside of North Korea) and 2) what's the alternative?
Internet security experts are of the opinion that this was launched by a large and well-organized group. That suggests we aren't dealing with a disgruntled employee, but with either a large criminal organization or a nation-state. This narrows things down considerably.
Next, let's look at motives. If the organization is a criminal organization, they're going to be out for one thing: money. As far as we know, there weren't any financial demands. The hackers said "if you don't obey us, then we'll release data shown below to the world", but they never mention money. The group's name- Guardians of Peace- is also telling, and there's the bizarrely moral tone of the hackers. "You, the criminals including Michael Lynton will surely go to hell. Nobody can help you". They are doing this for ideological reasons. Of course organizations like Anonymous also engage in politically motivated hacking, but they're usually upfront about the cause and the fact that it's Anonymous, which suggests it's not them.
Which brings us to North Korea. Again, it doesn't make sense... but this is a nation that reveres its dictators as gods and lives in a bizarre bubble of disinformation, lies, and communist mythology. Things that seem insane to us make sense in this communist Bizarro-world. Hacking Sony is bizarre, but this is a nation that starves, impoverishes and executes its citizens to maintain their grip on power... if they did it, hacking Sony was probably not even the craziest thing that happened that week in the country. And as for not wanting to provoke a war... these guys torpedoed and sank a South Korean naval ship killing 46 people. If that's not going to create a war, no way hacking Sony will. And the thing is, they actually *want* to go to the brink of war, but not quite over the edge into a war. If they can keep the tension ratcheted up they gain in negotiations with the outside world and can convince their citizens that the State is necessary to protect them all. It's like they're using 1984 as a manual: a state of perpetual warfare (or at least military readiness) provides a convenient pretext for anything the state does to exploit and oppress people.
Last, the attacks bear striking similarities to recent attacks against South Korea, down to the skeleton-themed graphics that look like they're from some mid-1990s video game console, the tacky red-and-green text, the poor English ("Warninig" instead of "Warning"), and the approach of taking user data hostage. It's pretty clearly North Korea.