iCal 1.5.4 [Updated]
iCal 1.5.4 for Mac OS X 10.2.3 or later is now available through Software Update or download.
iCal 1.5.4 includes security enhancements to help protect your computer when importing events with alarms that open files or applications.
iCal version 1.5.3 was apparently never released as a Software Update, the previous update having been the January 2004 release of iCal 1.5.2.[Update] - A Knowledge Base article describes the handling of "unsafe" alarms in iCal 1.5.4.