The URIBL (which, like SURBL - filters junk mail based on spam sites mentioned in their message bodies) website was rendered temporarily available by the assault between Wednesday and Friday. It used DDoS mitigation technology from Prolexic to restore services.
While the true intention of the attacks is not clear, it is obvious that each companies campaign against spam mail was a factor. The attackers may have used Storm worm variants, which have been discussed heavily for a while now, for the attack. The fact that the attacks were relatively ineffective is a testament to the evolving protection against said attacks and quicker responses, but also shows that they are still a thorn that must be dealt with. Nobody loves spam, and the communities fighting against it, whether for profit or volunteer, are welcomed.