Just in case you are one of the Mac OS X 10.5 (leopard) early adopters, and were feeling left out by all of the other updates, Apple has released Mac OS X 10.5.1. It contains a lot of the security fixes that came out yesterday for the other Mac OS versions; plus stability and functional enhancements. So you'll definitely want to install this as soon as you can.
For more information, see Apple Security Updates.