Mac OS X has been updated to 10.4.10, bringing with it several fixes. Among the fixes was included a security-related vulnerability, which if exploited could lead to denial of service attacks. The flaw in particular is not all that dangerous, and from the wording seems to only affect IPV6:

The IPv6 protocol allows remote attackers to cause a denial of service via crafted IPv6 type 0 route headers (IPV6_RTHDR_TYPE_0) that create network amplification between two routers.
Other fixes in this update include a patch for USB and bluetooth functionality, along with several generic "operating system fixes". Additional application compatibility was included and adjustments for RAW cameras was added as well. The entire update package ranges from a 25MB download to a 391MB download depending on what you are running.

This is the sixth patch release Apple has crafted this year, with them more or less beginning to fall into a monthly patch cycle. The increased popularity of OS X, in no small part due to Apple's introduction of Intel-based Macs, likely has prompted them to develop more. At least their users would hope.

You can download the patches yourself at their site if you so wish.