Apparently Apple included an older version of Adobe's Flash Player plugin with the latest Mac OS X 10.6.4 update. The version Apple supplies is 10.0.45.2, whereas the latest version from Adobe is 10.1.53.64. While most people will not experience much difference in terms of performance, there could be some stability or compatibility differences that a few people may notice. More importantly, the version included with OS 10.6.4 is one that is subject to some recently uncovered (and subsequently patched) security vulnerabilities.
According to a recent ComputerWorld article on the matter, the 10.6.4 update does not appear to downgrade an existing installation of the latest version of Flash, so people who have already updated should be fine; however, be sure to check your browser to see what version you have installed. This can be seen by getting information on the "Flash Player.plugin" file in the /Macintosh HD/Library/Internet Plug-Ins/ folder, or by choosing "Installed Plug-Ins" from the Help menu in Safari and checking the version.
If you are in doubt about what version of Flash you have, you can just run the Flash installer again to ensure your system has the latest version, even if you believe you already have the latest version. This may be good to do, regardless, because of the security holes in the version shipped with OS 10.6.4.