You can do it, but it seems to require a .mac membership.
From Panther's iSync 1.4 Help:
If you have a .Mac membership, you can use iSync to synchronize your Address Book contacts, iCal events and To Do lists, and your Safari bookmarks across multiple Mac OS computers using the Internet.
Before you can sync your computers, you need to register them with the .Mac synchronization service. You only need to do this once for each computer.