While unverified, this update may be a response to a recently-released jailbreak tool, which supposedly enables piracy on Sony's once-bulletproof system. We'll update this story if Sony provides any additional details on what this firmware does exactly.
Update: The BBC reports that the update does block PSJailbreak and PSGroove hacks. "Since this is an overall security related issue, we will not be providing further commentary to this case," a Sony representative said. "But as we always have, we will continue to take necessary actions to both hardware and software to protect the intellectual content provided on the PlayStation 3."