Yesterday, Apple released Security Update 2007-004, which fixes about 25 vulnerabilities in a wide variety of system components. Many of the vulnerabilities are quite serious; some were publicly disclosed during the Month of Apple Bugs in January. So if you haven't patched yet, now is the time; and 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.