Apple released Thursday Security Update 2009-0001 for Mac OS X, though the company is so far sketchy on the details.
The release notes in Software Update say, "Security Update 2009-001 is recommended for all users and improves the security of Mac OS X. Previous security updates have been incorporated into this security update."
Apple's Knowledge Base Web page that coordinates information on the company's security updates does not yet have the new update posted, so for now, that's all we know.
The update is a 44.1 MB download through Software Update for Mac OS X 10.5.x. A restart is required when installing the update.