I don't see any problem.
Anonymous has hacked quite a few sites in their existence. Good show on giving the world a chance to return the favour. And if the world can't do it, then it's a good way of demonstrating their security chops.
However, I sort of feel that the Anon Forum will be hacked sooner than later. It's a PHP bulletin board - no, not phpBB (which used to be the proverbial swiss cheese), but Zetaboards, which I've heard little of for good or ill. This Hacking FAQ does not fill me with confidence, mind you.
Its advice to choosing a strong password is good. However, sentences like "Get to the Admin [Control Panel]! Immediately get to find/edit/suspend user and demote every staff member you think has an account at risk and the account of the cracking admin" smells of panic.
I was hoping for stuff like "Zetaboard is hardened against CSRF and SQL-injection vulnerabilities", but that all seems to be missing in action.