This update upgrades Safari to version 4.0.2 as well as accuracy of full history search in Safari 4. Also some minor improvements and fixes within Bluetooth, iCal, MobileMe Sync, CalDav, AirPort networks.
You can read all the details about the 10.5.8 update at Apples website.
Before you install this new update, it’s probably a good idea to make sure you have an up-to-date backup, either with Time Machine, SuperDuper or your other favorite backup application.
Also I’d recommend you do a “Repair permissions” on your hard drive within Disc Utility.
You can get the update from the built-in Software Update from the Apple menu in the upper left corner of your screen or download the standalone installer.
I just updated to 10.5.8 on my unibody MacBook Pro and encountered no problems at all.