Apple has released a new version, 10·6·8 of the OS X operating system for the Mac, along with Security Update 2011-004. This update fixes 39 different identified vulnerabilities, some of them serious. Details of the vulnerabilities fixed are in the Security Update document mentioned above. The updates apply to both core system components, and popular applications, like QuickTime.
I recommend that you install the new version as soon as you conveniently can, though as always testing first is a good idea before installing any software update in a critical environment. You can get the new version via the Software Update service, or you can download it here.