Apple has released Security Update 2007-005 which fixes a number vulnerabilities in a wide variety of system components. Some of the vulnerabilities are serious, such as ones involving PDF files and Plug-and-Play, so if you haven't patched yet you should do so now. You will be required to restart.
To make sure your computer is up-to-date, you can run Software Update to check at any time. For instructions on how to run Software Update, please see Mac Maintenance on the Help Desk web site or Mac OS X: Updating your software on Apple's web site.