Apple has released a small update for its Software Update service, which addresses a problem with Snow Leopard systems where the installers for some software updates may not run. This issue does not happen for all software updates, so only a select few people will experience updates not installing; however, without being patched, the issue might surface with future updates that Apple releases.
On systems where this problem happens, the Software Update utility will detect updates as being available, but when you click to install them, nothing will happen. If you are having such problems on your system, then you might try downloading and running this latest patch. The patch should be available via Software Update for systems running OS X 10.6.8, so be sure you have applied the latest OS update in order to install this patch (don't worry, it does not require itself in order to install). In addition to using Software Update, the patch can also be downloaded from the update's Web site.
When installed, your system will have a new version of the PackageKit framework that is used for handling the .pkg installer files that are used to distribute software updates from Apple and third-party developers, and will also replace some launch agent and daemon files in the System folder. You should not need to reboot your system after applying this update; however, doing so will ensure the new launch daemons and framework executables load properly after updating.