The 10.5.8 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, as well as specific fixes for:

compatibility and reliability issues when joining AirPort networks.
an issue that could cause some monitor resolutions to no longer appear in Displays System Preferences.
issues that may affect Bluetooth reliability.

For detailed information on this update, please visit this website:http://support.apple.com/kb/HT3606.

For information on the security content of this update, please visit:http://support.apple.com/kb/HT1222.

Grab the Combo Updater if you’ve got a couple of machines to update.

There’s also some more information about exactly what was updated here: About the security content of Security Update 2009-003 / Mac OS X v10.5.8