I doubt it, this sort of thing has happened before. They have goaded people into suicide and at least once a murderer posted on there during the crime, even posting photos of the victim before he was caught.
In fact I'd delete the links above as I think the photos appear in that thread.
I've removed the links and any mention of the kid that did this.
Sadly I think that you're right about any repercussions, although some of the anons will have to live with their complicity for the rest of their lives. I think this does give law enforcement agencies their best opportunity yet to knock some lumps out of 4chan - the actions of the shooter were posted on their boards and they didn't respond, and other users encouraged acts of violence. The timeline of this event differs from the murder last year, which I understood to have taken place before he posted on the board?
In
US legal terms those users who encouraged this shooter were accessories, actively involved:
(a) Whoever aids, abets, counsels, commands, induces or procures the commission of an offense, is punishable as a principal
(b) Whoever willfully causes an act to be done which if directly performed by him or another would be an offense, is punishable as a principal
They'll need some decent IP masking. 4chan will need some shit hot lawyers.