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.