The Mac OS X 10.5.8 update is now available through Software Update. Among the listed features are resolution for compatibility issues when joining AirPort networks, disappearing monitor resolutions in System Preferences, and Bluetooth reliability.
The update will also upgrade Safari to version 4.0.2, though I'm not sure if that includes those of us still on Safari 3 (I'll find out in a moment!) There are a number of other reliability and compatibility tweaks that are listed here. Security features are available here.
The update is not available through Apple's download site as of yet.
Edit (1:55 PT): I can confirm that this update does upgrade those of you still on Safari 3, like I was, to Safari 4.0.2. The combo update is also now available.