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The update addresses one major problem - where saving Adobe CS 3 files saved to a remote server could result in data loss - as well as issues with iCal, AirPort, Safari support for secure web pages, Spaces, Exposé, VPN and X11.

Of those iCal gets the most attention, with improvements to overall reliability for meeting requests, cancellation notices, delegation and syncing with iPhone among the changes.

Apple has also provided

The update also includes fixes for a number of security vulnerabilities. Notably Exposé hot corners can no longer be used to bypass a sleep or screensaver password and WebKit's handling of JavaScript arrays no longer exposes users to the risk of unexpected application termination or arbitrary code execution.

Mac OS X 10.5.4 is available via Software Update or from apple.com/support/downloads. For more information see About the Mac OS X 10.5.4 update and About the security content of Mac OS X 10.5.4