Apple has released OS X 10.5.8 for Leopard. The update includes a generic list of fixes:
Upgrades Safari to version 4.0.2.
Improves the accuracy of full history search in Safari 4.
Resolves an issue in which certain resolutions might not appear in the Display pane in System Preferences.
Dragging an Aperture image into Automator now invokes an Aperture action instead of incorrectly invoking an iPhoto action.
Resolves an issue that could prevent importing of large photo and movie files from digital cameras.
Improves overall Bluetooth reliability with external devices, USB webcams and printers.
Addresses an issue that could cause extended startup times.
Improves iCal reliability with MobileMe Sync and CalDav.
Addresses data reliability issues with iDisk and MobileMe.
Improves overall reliability with AFP.
Improves overall reliability with Managed Client.
Improves compatibility and reliability for joining AirPort networks.
Improves Sync Service reliability.
Includes additional RAW image support for several third-party cameras.
Improves compatibility with some external USB hard drives.
Includes latest security fixes.
On a side note related to my problems with OS X 10.5.7: I restored the setting that caused the lock-ups and was immediately able to cause the lock-up. Then I did the update and have so far been unable to cause the lock-up.
So far the update seems to be fine on my iMac, but recent experience has me keeping my fingers crossed.