Being the paranoid person that I am, it occurred to me that it would be a good idea to back up Address Book and iCal before all of the syncing madness begins between Macs, MobileMe, "the cloud", and iPhone/iPod touch. Here's how to do it (Mac OS X 10.5 Leopard only):
Backing up Address Book
Launch Address Book
Select File > Export > Address Book Archive
Find a location to save the Address Book archive, then click the Save button.
Backing up iCal
Select File > Backup iCal
Find a location to save the iCal backup, then click the Save button.
Backing up iPhone / iPod touch
Just sync it
Ahhh, don't you feel all safe and happy now? And remember, if you're running Leopard and Time Machine, your backups are going to be backed up, too!