The al-Shamukh site has been around for a while now as a sort of message board forum for members of al Qaeda and other terrorist organizations. It bounces around from server to server but has never been publicly targeted for take down because of the potential intelligence information it could reveal. Now it's been knocked off line in what is being described as a very sophisticated attack. The attack seemed to go after both the site and the server that supported it. The attackers may have gained control of URLs and redirected them elsewhere to be disruptive.
Now. Who could launch sophisticated attacks? LulzSec? Anonymous? Perhaps but this could very well be the work of one or more governments fighting terrorism in a way that will likely become much more common going forward.