The North Korean theory seems silly. In fact, it's exactly what I might say publicly if I were Sony and I wanted to try and turn lemons (being hacked) into lemonade (free buzz about an upcoming movie). The movie Kim Jong-un doesn't want you to see! In the end, I doubt there was any hacking involved at all: a disgruntled employee leaked documents. Perhaps Sony forgot to disable someone's password after giving them the…
Unfortunately we'll probably be left guessing as to how this hack happened and how long it went on, since Sony is not likely to ever release that information even if they figure out the cause.