Did you know that you can backup your address book and your iCal?

Begin by making a backup of your iCal and Address Book data. I explain how to back up iCal toward the end of this entry. To back up Address Book, in Tiger choose File -> Back Up Address Book and in Leopard choose File -> Export -> Address Book Archive. Should you need this backup you can use Address Book's Revert (Tiger) or Import (Leopard) commands to bring back your data. 

For iCal, do the same thing from the iCal menu.