Second, this flaw is just nasty, affects nearly every running version of Windows, is remotely exploitable, and could very well end up seeing a worm developed to target it. Time was of the essence for Microsoft to release this patch, and now time is of the essence for you to deploy it.
Microsoft Security Bulletin MS08-067 explained that this vulnerability resides in its Server service, and attackers could create specially crafted remote procedure call requests either for one-off exploits or an outright self-replicating worm.
According to Thomas Claburn's story from earlier today, attacks were already in the wild. Quoting Jason Miller, security and data team manager at patch management software maker Shavlik Technologies:
"This vulnerability is pretty nasty," said Miller. "It affects every version of the Windows operating system. Attackers don't need to have any credentials or access to the machine. They don't have to trick the user into doing anything."
His biggest fear, he said, is that a worm will be developed to take over vulnerable machines en masse. And he fully expects that to happen. "You're talking about a vulnerability that does not need user interaction," he said. "That's a gold mine if you're trying to build a botnet."
Sounds like a good enough reason for an emergency patch to me, and a great reason to test and apply ASAP.