With previous versions of iCal I was used to backing up my calendars by copying the iCal folder from my user's Library/Application Support folder.
After performing an erase and install of 10.5, I launched iCal to see what the new version looked like. I then copied my iCal folder from my backup to the location shown above and launched iCal again. I was shocked to find that my calendars were simply not recognized by the new version of iCal. I later found out that iCal 3.0 stores its information in my user's Library/Calendars folder. I found that I could force iCal to convert my calendars by doing the following:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071108231407248