This just showed up in my System Updates

The 10.5.1 Update is recommended for all users running Mac OS X Leopard and includes general operating system fixes that enhance the stability, compatibility and security of your Mac.

For detailed information on this update, please visit this website: http://docs.info.apple.com/article.html?artnum=306907.
For detailed information on security updates, please visit this website: http://docs.info.apple.com/article.html?artnum=61798.