Joe,
If you make the backup using iCal>File>Back up iCal..., you will have a file named something like iCal 2008-12-30.icbu.
Using iCal>File>Restore iCal..., navigate to the file, and follow the prompts to restore from your iCal backup file as it existed when the backup was made. In the example I gave, which was my last backup, iCal would revert to the state that existed on Dec 30th, and any entries made since that date would not be preserved.
If you made the backup using Time Machine, restore the Macintosh HD/Users/yourusername/Library "Calendars" Folder from your most current Time Machine backup.
;~)