Early versions of the iCal calendar program from Apple placed calendar files separately (for each schedule) into the Library \ Calendars folder. So, backing up the calendar files was as easy as copying these to another location.
The current version of iCal (version 2.0.2) that comes with Tiger no longer uses this method. Instead there is a database that holds all calendars.
Individual calendars (schedules) can be imported or exported. However, to backup the entire database, it is necessary to choose "Backup Database..." from the File menu.
More information about iCal can be found here: