Lol you people do realize that it is highly unlikely that the users of the sites he got the IP's from were using a proxy or whatever to mask their IP every time they used the site? He probably took the ones that were the most common appearing of the users and posted those.

Oh and just because you use a proxy doesn't mean you can't be tracked. A good hacker can track an IP through 100 proxies if he knows what he is doing. You can always ALWAYS trace something back to the source, especially when it comes to the Internet.
Trust me if Sony or the FBI wanted to find these guys, they would find them simple as. You really think these people are smarter than the FBI?(Or whoever would be hired to track them down). They may not be amateurs, but they would obviously not be the smartest out there.

And before you come at me with 'you can erase your presence from anywhere on the internet', that's untrue. Someone could always know a different way of tracking you down that you are unaware of or don't know how to cover up.